Add basic support for linking to local files

For the case when you want a PDF table of contents linking to a directory of local files that can be passed around on a flash drive.  Old school.

When using Prawn's URI support, the file is opened in a browser.  But, section 12.6.4.5 of the PDF spec allows for 'Launch Actions' where a local file is opened using whatever native application is available (eg. a .doc will open in Word, a .xls in Excel, etc).

This code allows the following to be passed in the formatted_text hash:

:local =>  './relative-link-to-local-file'