log: Add verbosity_level 2 and 3

Reduces the amount of dependency logging on level 1, and increases the amount
on level 2 and 3. Fixes #593.
log: Remove default config that has no effect for verbosity_level < 2
docs: Explain -v/-vv/-vvv differences
docs: Update changelog
Merge pull request #638 from jodal/feature/advanced-log-levels

Support advanced log levels with -vv and -vvv