Add --ext argument

This release includes the ability to listen to other extensions than the /\.rb$/ regex.

For now, this extension is really just useful for hardcoded commands. An example of usage can be:

$ retest 'go test' --ext=\\.go$

This is a first step towards opening retest to other programming languages.

Note: Using placeholders e.g. <test> will probably not work as finding test files is still coupled to ruby extensions. We'll look into changing this in future releases.