Improve generation performance by not blowing the cache for every input file.

HandlebarLoader keeps a cache of mustache templates to speed up resource loading.
This makes the cache static over multiple input files.

Timing the old code
$ time java -jar $oldjar -v  -l scala $files &> /dev/null

real	0m15.544s
user	0m20.705s
sys	0m0.758s

Reusing the cache:
$ time java -jar $jar -v -l scala $files &> /dev/null

real	0m5.239s
user	0m8.884s
sys	0m0.360s

TODO: add benchmark scripts.
well, its the standard issues that come from global state. 
http://c2.com/cgi/wiki?GlobalVariablesAreBad

in this case, i think it would be easy to just pass in the same HashMap to the HandlebarLoader class constructor. if this is difficult, i'm fine with leaving as is for now and revisiting later.