add 'zigup latest' command

[travisstaloch]

adds a command which sets the default compiler to the current local master version without downloading anything.

closes #110

I'm not sure this is the best solution as it requires a 'master' version to be available. But it does the job for my most frequent use case: switching back from 0.11.0 to some 0.12.0-dev version without downloading anything.

I haven't added any tests, only verified that it works locally. Let me know if is should add any tests.