Add arguments for iz and izz to show only ascii unicode or wide strings

[alex-bender]

Description

Allow iz and izz commands to show only requested type of the strings:

• a for ascii
• u for unicode
• w for wide strings

[trufae]

Bear in mind that now rbin supports cfg.charset for a larger set of encodings like for gameboy roms or MVS objects with ebcdic37, so there must be also a way to specify the full name for the encoding. maybe iz:pokered or iz:utf8 and such

[trufae]

I think we should just use rtable in cbin