Change verbose mode to a verbosity level for the command line options

This is a patch to help fixing #41.
Should the default verbosity level stay the same as the previous `verbose` mode (i.e. the default stays `--verbosity 0`), or should we change it to `--verbosity 1` (i.e. we display `Loading component: componentX` lines by default)?
If it can't parse an integer, it returns `NaN`.
```
> parseInt("adf", 10)
NaN
> parseInt("asdf", 10) > 0
false