core: add comment with return value of function string_regcomp
core: fix typo in french translation of /help upgrade
doc: fix emphasized text in plugin API reference and relay protocol
core: remove obsolete enum t_config_look_nicklist
core: add text emphasis in messages when searching text in buffer

New options:
- weechat.look.emphasized_attributes
- weechat.color.emphasized
- weechat.color.emphasized_bg
core: add search of regular expression in buffer, don't reset search type on a new search, select where to search (messages/prefixes)

Key changed in search context:
- ctrl+"r": switch search type: string/regex

New keys in search context:
- alt+"c": case (in)sensitive search
- tab: search in messages/prefixes/both
doc: capitalize first word of text in table cells