Check for "Re:" at start of subject in case insensitive manner

To avoid a subject ending up like:

    "Re: RE: RE: RE: original subject"

That I've seen a few times.

The fix is to lower case the subject and the strings to compare against
before calling startwith()
Merge pull request #660 from foobacca/case-insensitive-reply-prefix

Check for "Re:" at start of subject in case insensitive manner
On retagprompt, append a comma

So we don't have to remember to type the extra comma before typing a new
tag.
in retagprompt, sort the tags

So the tags are in a predictable order.
retagprompt: skip empty tags

(and so avoid having a leading comma)
Merge pull request #659 from foobacca/improve-retag

Improve retag
add --tags option to global taglist command

that sets a whitelist of tag strings to be displayed
cf issue #662
Merge branch '0.3.5-feature-taglist-filter' into testing

Conflicts:
	alot/commands/search.py