Add custom var mu4e-view-html-plaintext-ratio-heuristic

Add a custom var to enable the customisation of the ratio between the length of the html and the plaintext parts under which the text is considered to be garbage
* add mu_str_replace
Merge pull request #224 from tuxella/master

* allow the customization of the plaintext / html length ratio
* mu-cmd-server: add support for moving without renaming
* fix CSV quoting (fields with commas)
* mu4e-compose: unset the new flags when forwarding/replying to messages; fixes #214.
* test-mu-maildir: fix regexp
* fix matching backslashes, add some tests
* fix quoting of contacts in autocompletion (thanks to jhelberg)
* mu4e: set overwrite mode to nil in mu4e-headers-mode
Fix typos.
Merge pull request #229 from sje30/master

* fix typos in mu-find.1
* merge branch 'master' of github.com:djcb/mu
* mu4e: add mu4e-change-filenames-when-moving customization