use regex library for recursion support

[solaris007]

for recursion support it would be awesome to use the regex instead of std lib.
this would introduce a pip dependency though. not sure how to handle this?

recursion support would be nice to capture and color nested parenthesis lines, e.g.:
27.03.2019 10:42:27.275 *INFO* [FelixStartLevel [Thread-232]] org.apache.sling.commons.threads.impl.DefaultThreadPool Thread pool [DAM Transporter] initialized.

Using regex: \[([^\[\]]+|(?R))*\]

Which matches: [FelixStartLevel [Thread-232]] and [DAM Transporter]

[garabik]

Adding conditional import of regex depending on module availability is easy and I might just do it - I am just a bit concerned that grc fails when using such an incompatible config file.