Predefined command option for VimuxPromptCommand

Added the possibility to specify a predefined command which
can be edited or completed in the prompt line.
Removed Debug Output

Removed debug output of command which was still in there
Updated documentation for VimuxPromptCommand

Added description for VimuxPromptCommand optional parameter.
Merge pull request #113 from m42e/master

Predefined command option for VimuxPromptCommand