Support logoutput option

Installing all dependencies can be long on large projects. If you're in an environment timing outputs to detect stalled processes, puppet-bundler can look like it is stuck.
One can also be interested in detecting the slowest gems to install.

This changeset brings support for providing a `logoutput` option, possibly allowing outputting a line per gem.
The default value stays at `on_failure`, the previous default. Other allowed values are `false` or, more interestingly for the use-case above, `true`.