simplify completion code by merging {Base,Native,Custom}PathCompleter classes into one class
some logging
move shell command completion code into a new ShellCommandResultCompleter class
added support for custom path completion using an external command
Remove duplicate code to read path_completion_command setting

The instantiation of the appropriate *PathCompleter instance is
now done automatically by the PathCompleter class which will read
the path_completion_command setting, instantiate the good class
and proxies the completion request to the good completer.
make sure we don't use CustomPathCompleter for attachments or threads saving
simplify completion code by merging {Base,Native,Custom}PathCompleter classes into one class
move shell command completion code into a new ShellCommandResultCompleter class
fix native path completion
Merge branch '0.3.5-feature-custom-path-completion-683' of https://github.com/yannrouillard/alot into 0.3.5-feature-custom-path-completion-683

Conflicts:
	alot/completion.py