Minor refactor pulling out git subprocess commands into helper functions
Rewrote the getargs method to also locate the source directory for copying attachments from.  Note that this changes the command line arguments for the program to now take the directory for trac and the directory for the git repo.
Refactored the formatted page name so its reused explicitly (I'll need this later for attachment paths)
Added an array containing details of the attached files to the page information.

We add an empty list for all previous versions of the page and then
populate the list for the final one.  The idea is that the processing
code will copy each attachment over and add it to git as we process each page.

Note the use of urllib.quote in order to discover the filesystem name
which trac gives to the filename
Work through the list of attachments for a page and copy them over,
adding them to git as you go.
Add a list at the end of the new markdown page with a link to each attachment in it
Added a formatting rule for dealing with trac inline images.

This required an extra argument to the format_text method so that I can
build the path to the attachments