* Add erlfmt fixer to the registry
Without this, the fixer will not appear in the list of suggested tools
and cannot be used without additional configuration.
* Handle stdin in the erlfmt fixer command
Previously, the full path to the file being edited was used, which
resulted in the loss of unsaved changes.
* Add executable selection tests for erlfmt fixer
Most of the time it works to assume that the current working
directory is the root of the project. However, this is not the case
for Rebar3 checked out dependencies, for example.
It's also worth noting that because of the way Elvis handles file
patterns, and because directories in configuration are relative to the
project root, the path supplied to command must be also relative.
