Full mirc colour+style compliance

Tests are on this branch, just remapped the classes `irc-fg{colour}` to `color-{colour}`, and similar for background `irc-bg{colour}` to `bg-{colour}`

Adds support for italics

Underline, bold and italic are now placed in a span

https://github.com/megawac/irc-style-parser/tree/shout

Resolves #257, closes #304, fix #210, fix #264, fix #216

[Travis test summary](https://travis-ci.org/megawac/irc-style-parser/builds/43581604)