Protect existing solutions from being overwritten by 'download'

[haguro]

Add an optional --force flag to the download command to overwrite an existing exercise directory as suggested in several issues and discussions.

Should fix exercism/exercism#5432 and probably #872 too.

Feedback or further guidance is much appreciated 👍

[haguro]

The travis builds for Go 1.11 and 1.12 are failing due to compile errors in the testify package. Building with Go latest (and locally Go 1.15) is successful. Is there a reason for the cli to be buildable with those old Go versions?

[iHiD]

@haguro Thanks :) I don't speak Go so this will require @ekingery taking a look :)

[ekingery]

Code and tests look great, thanks so much, @haguro! If you are interested in additional contributions to the exercism, we have plenty of work to do on the Go track to get ready for the V3 launch. Let me know if so!

[haguro]

Many thanks @ekingery. Yes, I would be happy to lend a hand!

[ekingery]

Many thanks @ekingery. Yes, I would be happy to lend a hand!

Excellent! If you would like a slack invite, send me your email address (mine is my username at gmail). If you don't like Slack, I'll just add you to an issue or two once the work is ready to be distributed.