Let user force handler when choosing between ambiguous extensions

[Miss-Inputs]

A continuation of #13 but I'm setting it aside for after 0.5.

This will probably require extending chooseChoices so I can add an additional button that gets placed on the generated form, and that button brings you to the list of all handlers to force.

Alternatively, a checkbox that toggles between showing handlers that handle the given file extension, or all handlers.

Either way I'm not getting out of modifying chooseChoices.