Add lexer for colon-separated value config files like /etc/passwd, /etc/shadow and /etc/group

[Leistungsabfall]

Hi,

I created two lexers (PasswdLexer, ShadowLexer) for lexing linux config files /etc/passwd and /etc/shadow.
I don't know if they are of much use for pygments though, so feel free to reject this PR if they are too specific.

/etc/passwd:

/etc/shadow:

[jeanas]

I don't know if they are of much use for pygments though, so feel free to reject this PR if they are too specific.

They don't seem egregious to me, but I would prefer if they didn't match files named passwd and shadow.

[birkenfeld]

It seems to me that they could be combined into a single lexer, with highlighting determined not by position but by content...

[Leistungsabfall]

Thank you both for your valuable feedback.
I applied the suggested changes and merged both lexers into a generic UnixConfigLexer that is able to parse any config file using colons (:) as value separators (e.g. /etc/shadow, /etc/passwd, /etc/group).

I'd appreciate if you could take another look at the changes :)

[jeanas]

Sorry that I forgot to say it the first time: this PR is missing some tests. Please add them as tests/snippets/unixconfig/test-name.txt and run pytest --update-goldens tests/snippets/unixconfig to generate golden test output.

[jeanas]

Looks OK to me. Leaving this around in case @birkenfeld wants to take another look.

[birkenfeld]

Thanks for the contribution!