Suggest stackoverflow sources when they are not found & list available sources

[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.

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