(Slack) Attachments handling

[jaroslawp]

This PR adds (slack) attachment handling

• should address https://github.com/EionRobb/purple-mattermost/issues/12
• I think we do best we can with limited xhtml-im libpurple capabilities ...

Not handled:

• multiple colums in attachments (just displayed in single column)
• symbolic color names ("green", "warning" .. etc)

Please check with libpurple-3 (the util.c libpurple xhtml-im handler is really fragile .. easy to get non-htmlized output even while using a sequence of valid html tags ...)

And this is how it looks like:

[jaroslawp]

that was blazing fast ;-)

[EionRobb]

Sorry, won't happen again, I promise!

[jaroslawp]

pity ;-) other 2 PRs are awaiting ;-)