Allow chat history (selection) to be stored as a txt/cvs file

[putt1ck]

As per comments on #35:
#35 (comment)
&
#35 (comment)

It would be useful to have the ability to export/copy a text chat history or part thereof and have it associated with a file or folder that is created as a result of the discussion, possibly as comments.

[bes1002t]

But imho this should be also encrypted on the file system.

[nickvergessen]

@jancborchardt are you okay with putting it as an option into the "…" menuof the conversation list?