Several enhancements: (b)atch and (c)continue commands; relative address ranges, ascii dump option; compact mpu status; doc updates

[patricksurry]

py65 has been super useful while taking apart an old atari game (see work in progress https://github.com/patricksurry/eastern-front-1941/)

This PR collects a number of small extensions and improvements I made to py65 as I went along:

• (b)atch command: I had a bunch of setup commands for labels etc that I repeated on each launch. I initially did cat setup.cmd - | py65 which sort of works but it doesn't echo the input lines making the input confusing. The batch command and command-line option reads a file to the existing cmd.cmdqueue buffer and tracks whether to echo each command as it's executed, as if typed interactively.

• (c)ontinue: to continue from current PC to next breakpoint, also suggested in https://github.com/mnaberez/py65/issues/51 I prefer continue to go based on other debuggers and go feeling too close to goto. Another option would be to support goto without an address

• support for implied and relative address ranges. Omitting the start address in a range continues from the end of the last address range. Writing a range with a / separator interprets the second value as a length rather than an absolute address. This works for disassemble, fill and mem making it easier to step through chunks of code/memory. Hitting enter to repeat a last command like d /20 will disassemble the next 20 bytes (aligning to the end of the last instruction).

.m 2100:2120
2100:  a9  00  85  82  a9  04  85  83  a9  00  85  84  a9  06  85  85  a9  1c
2112:  85  80  a9  21  85  81  4c  23  20  00  14  22  03  00  af

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>
.m :2140
2121:  22  1b  21  00  00  04  45  58  49  54  2d  21  38  a5  84  e9  02  85
2133:  84  b0  02  c6  85  a0  00  b1  84  85  80  c8  b1  84

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>
.m 2140/20
2140:  84  85  81  4c  23  20  24  21  04  44  52  4f  50  4f  21  38  a5  82
2152:  e9  02  85  82  b0  02  c6  83  4c  23  20  46  21  04

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>
.m /20
2160:  53  57  41  50  66  21  38  a5  82  e9  02  85  82  b0  02  c6  83  a0
2172:  00  b1  82  85  86  c8  b1  82  85  87  38  a5  82  e9

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>
.
2180:  02  85  82  b0  02  c6  83  a0  00  b1  82  85  88  c8  b1  82  85  89
2192:  a0  00  a5  86  91  82  c8  a5  87  91  82  18  a5  82

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>
.

• optional ascii|noascii option for the mem command to append an ascii representation of each line. This is useful for understanding string tables etc:

.m 2290:22c0 ascii
2290: 23 20 3a 22 06 44 4f 55 42 4c 45 00 20 c7 21 3e 22 2b  # :".DOUBLE. G!>"+
22a2: 21 92 22 09 51 55 41 44 52 55 50 4c 45 00 20 9b 22 9b  !.".QUADRUPLE. .".
22b4: 22 2b 21 a9 00 88 10 01 60 51 86 0a 69                 "+!)....`Q..i

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>

• updated doc including a note about ;-style comments I discovered but hadn't seen written anywhere.

• as a personal preference I compacted the MPU summary from two lines to one which imho makes it much easier to compare changes as you step thru code. This could also be a command-line switch I guess? Here's the before and after:

       PC  AC XR YR SP NV-BDIZC
6502: 0000 00 00 00 ff 00110000

6502: PC=0000 A=00 X=00 Y=00 SP=ff FLAGS=<N0 V0 B1 D0 I0 Z0 C0>

[patricksurry]

lmk if it'd be helpful to break this PR into smaller chunks, since it combines a number of small features which maybe have individual objections