Skip directories like node_modules from the watchlist

Skip directories like node_modules from the watchlist

A local `node_modules` directory can easily contain
tens of thousands of files, easily exhausting the tiny
default max open files limit especially on OS X Yosemite,
in spite of the fact that  Hugo already had code in place
since February 2014 to try to raise the maxfiles ulimit.

Also skip `.git` and `bower_components` directories.

The file watching situation will improve when
https://github.com/go-fsnotify/fsevents become ready,
but until then, we will be thrifty.  :-)

Thanks to @chibicode for the suggestion.

See #168 for continued discussions.