Ensuring that newly prepended view extension / engine mappings override existing ones.

Signed-off-by: Ben Corlett <bencorlett@me.com>
Ensuring that existing extensions in the file view finder are also replaced.

Signed-off-by: Ben Corlett <bencorlett@me.com>
Refactoring file view loading to respect the non-greedy order in the environment. The newest registered views will be the first found. Actually fixes #506.

Signed-off-by: Ben Corlett <bencorlett@me.com>