Fixed bindings of entries in mu4e's header menu.
* mu4e: make the +/- in +tag, -tag only match at the beginning of words
* mu: fix pre-condition warning when getting related messages without threads
Merge pull request #160 from tmalsburg/master

* fix issue #159, broken bindings in the headers menu of mu4e.
* mu: give a bit more info when there's no mu script (guile) support; fixes #156
* fix typo