Add cmd line option for verbosity
Right now, the only way to set verbosity is to either set `verbose` or `debug`, which gives no option set lower output levels. This is fine in normal cases, but sometimes I don't want to see `Verifying dependencies ...`, etc. For example, if I care about the output of `nimble c ...`, then having all this noise is sometimes not preferable.

I propose that nimble have a `verbosity` flag just like the Nim compiler instead of `debug` and `verbose` which would allow setting the output levels to the desired amount.