Configurable number of spaces for indent

I'm a big fan of `npm install --save` but tend to use 4 spaces for indenting (whereas npm uses 2).  I added a config option (`indent`) which allows this to be changed.  If this is something you're interested in merging into npm I'll submit another pull request to npmconf setting the default `indent` value to 2 (and eliminate all the `npm.config.get('indent') || 2` here).