pprint task for debugging JSON config files

Hi,

lein-npm has a useful task (`lein npm pprint`) for showing the contents of transient, generated config files when debugging problems.

This PR adds a similar task for lein-bower: `lein bower pprint`

Example output:

```
lein-bower generated bower.json:

{"directory":"resources/lib"}

lein-bower generated .bowerrc:

{"name":"foo","description":"My Application","version":"0.1.0-SNAPSHOT","dependencies":{"react":"0.8.0","backbone":"1.1.0"}
```