Hero should only work on hero func files

[spekary]

Hero currently processes all of the files in a directory. Pure html files are compiled to blank .go files, and parent files are compiled just to a package name and nothing else.

Instead, I propose hero do two passes when running:

1. Examine all the files, looking for files that have <%: tags, and any other files that it determines it will compile to actually contain .go code.
2. Only process files that have been found and recorded, and leave all other files alone.