Allow Rails applications to override default views

The first time a template is rendered, Vanity checks to see if the path
'app/views/vanity' exists, and if so will load templates from that
location. Otherwise templates are loaded from the gem as normal.

This commit also adds a Rails generator to put copies of the original
templates into 'app/views/vanity'.
Add `custom_templates_path` configuration option.

Vanity.playground now has a configuration option,
`custom_templates_path`, which is used as a search location for custom
templates (to override the default ones in the gem).

By default this option is set to `app/views/vanity` in order to work
with Rails.
Merge pull request #229 from rsslldnphy/override-templates

Allow Rails applications to override default views