Suggest stackoverflow sources when they are not found

[lfoppiano]

This PR add a small change that allow to match the correct name when the provided name is not found:

Downloading and processing stackexchange dumps for ['portuguese.meta']
StackExchange source not found. Perhaps you meant ['portuguese.meta.stackexchange']

I've also added an additional command --list that shows the list of all the possible sources.

[CLAassistant]

All committers have signed the CLA.

[lfoppiano]

Closing this, opening #4 that pulls directly from a branch in my repo.