feat: add support for regexp rules

[martinohmann]

This implements the nice-to-have mentioned in #1.

In addition to matching and replacing image prefixes, this adds support
for using regexp patterns in exclusion and replacement rules.

This should allow to do any possible image transformation. When using a
regexp replacement rule, the replacement pattern can reference
submatches via $N where N is a number or ${name} where name is
the name of a named capture group (e.g. (?P<tag>.+)).

[pesarkhobeee]

I couldn't agree 100% percent with the logic. If we are going to support regex so why we should have our own way of defining rules too?
What I mean is the below code is unnecessary and repetitive
config.Pattern{Prefix: "someregistry.org"}
when I can do this
config.Pattern{"^someregistry.org"}