Add more path and directory meta config options

[colinschmidt]

Right now prependlocal enables easy relative path config variables. Adding a prependabs option that prepends the absolute path would be useful. In addition, another option that would help for absolute paths would be something like abspath that converts a path from relative to absolute.

[edwardcwang]

Idea: specify relative paths using a special meta variable? e.g. ${localdir}/my_file

[edwardcwang]

Resolution:

• prependlocal meta <--> vlsi.builtins.localpath
• prependcwd meta <--> vlsi.builtins.cwd

@colinschmidt re. abspath / prependabs these seem ambiguous to be as to whether we're working relative to localpath or cwd. Thoughts?

[edwardcwang]

Discussion from #232 says maybe localpath should be the default

[jwright6323]

We have a method of doing relative paths now, so this is not medium priority anymore.