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
Fix missing space in error message
crypto: eliminate invalid keys for gpgme.ERR_AMBIGUOUS_NAME in get_key

Previously when Context.get_key raised gpgme.ERR_AMBIGUOUS_NAME crypto.get_key
would not verify the ambiguous keys are actually valid for
signing/encryption.

Now when gpgme.get_key raises ERR_AMBIGUOUS_NAME we elimitate those invalid
keys. If there is only one valid key left we return it, otherwise re-raise
Improve documentation for crypto.get_key

Expand docstring and explain handling of ambiguous keyid
Merge branch '0.3.5-expired-keys-654'
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'
canceling a command in a sequence will now stop the subsequent commands from being run

Conflicts:
	alot/commands/globals.py
	alot/ui.py
integrate sequencing of commands into ui.apply_commandline

and obsolete commands.globals.CommandSequenceCommand
remove obsolete CommandSequenceCommand
docstrings for ui.appli_commandline
add missing return statement in RetagPromptCommand

cf #629
hand Command instance to pre/post hooks

When a pre or post command hook is called it gets
the Command instance it belongs to as keyword parameter "cmd".
This allows e.g. the post hook of TagCommands to access the
query string used in this particular instance.
doc: mention cmd-keyword parm to pre/post cmd hooks
re-raise CommandCanceled failures