Add hook to _load_EE_TMPL to allow substitution of EE->TMPL

This is primarily for compatibility with BiopsEE, but can be used
for any addon that extends the EE_Template class and which expects
Stash to use it.
Merge pull request #56 from EpocSquadron/biopsee

Add hook to _load_EE_TMPL to allow substitution of EE->TMPL