Don't block parsing files inside the wp-content folder

Currently, WP Parser is meant to be run in the root directory of a
WordPress site to generate documentation for WordPress core, and
therefore ignoring everything in wp-content makes sense. However, this
makes it harder to generate the JSON in a specific directory -- say for
a specific plugin.

One solution is to require people to check out a copy of that plugin
outside a wp-content folder, and run the documentation there. However,
as WP Parser is hopefully going to be used by all great plugins, this
puts a speedbump in the way of the plugin authors.

The onus should be on the person running WP Parser to decide which
directories/files need to be looked at, rather than blocking all
wp-content paths.