strip_colour: Strip '\x0F' (^O, formatting off)

Reported by ssbr on freenode:

chmode +c doesn't strip ^O, which turns off all previous formatting.
This can cause clients that internally use mIRC formatting to render messages weirdly,
e.g. highlighted messages in HexChat: <https://i.imgur.com/eDX8Aif.png>.
Merge branch 'patch-1' of https://github.com/auscompgeek/charybdis