add support for rendering templates from files
oops, forgot that this was changed back to how it was before
add explicit check to see if the template file exists

JTwig does this too, but I would rather the thrown exception be a
FileNotFoundException then a JTwig one
add tests for rendering from a file
add shared jtwig function repository object
add helper function to check if a twig function has already been defined
add macro for defining custom JTwig functions
pass in our JTwig functions repo object in to all new context objects
add doc comments and minor cleanup
move jtwig context related option handling to make-context
add functions to allow resetting of the jtwig function repository
change visiblity/name of the fn to check if a template fn has been added
lots of custom template function related tests