Expose settings holding the filtered command line arguments

This allows to change the filtering arguments at "runtime". For example,
the following binding allows to select a branch in the refs view for
filtering in the refs view:

	bind refs @ :set rev-args = %(ref)

Fixes #306
Add workaround for git pretty.format settings that break the log view

This overrides any pretty.format setting that contains color definitions
to use the 'medium' format. The fix adds the setting 'log-options' for
implementing the workaround. The option can be configured in `tigrc` and
works similar to `blame-options`. Finally, in order to test the feature
the semantics of `:toggle <args-option>` (note no arguments) has been
changed to simply clear the entries in the <args-option>.

Fixes #225