lib: move get_custom_lexer from utils to pygmentsutils

get_custom_lexer is the only dependency from helpers to utils.
In attempting to get a clearer dependency tree, we can move out
get_custom_lexer to a different place so that helpers does not depend on
utils.

It so happens that there already is a pygmentsutils.py file in lib, which is
a very good fit, since the lexers used in Kallithea are effectively provided
by pygments.