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.