(2520) Environment loaders now supports get_environment_dir

This is implemented only for directory based environments and returns
the path to the environment directory of the given environment.
Other loaders doesn't return any valid information (ie nil).

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>
(2520) Implements manual directory environments

Those are created with an environment_timeout value of "manual".
Those environments are cached indefinitely unless their directories
are manually touched (ie its ctime must change).

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>
(2520) support TTL unmunging for displaying the original value
(2520) add a face to list environments and flush 'manual' ones

This face allows to:
* list defined environments (including details with --details)
* flush one,several or all environements (this touches the environment
directory).

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>