Add a default message filter.

* Saved to localStorage['filtered-types-default'].
* Clicking a filter type while default is checked will uncheck it.
* Checking default will save localStorage['filtered-types-default']
  and delete localStorage['filtered-types-buffer-' + bufferId]
* Unchecking default will copy the default filter (or "[]" if unset)
  to the buffer filter.
Merge pull request #13 from Zren/defaultfilter

Add a default message filter.