Fixes #267

Issue: If I create a Mustache lambda to return numbers and I expect 0 to be of
those numbers, I'll instead only receive an empty string for 0 because it
evaluates to falsy.

Fix: Check the result of the lambda to be undefined or null. Only then should
it return the empty string.

Fixes #258

Fixes #247

Text tokens now accurately reflect their content's exact position
in the original template.

Fixes #187

Fixes #186

Fixes #42

Also fixed a bug that occurs when using the triple-stache with a key
that contains a closing stache. Fixes #192.

- Cleaner separation of responsibilities in the code between
  scanning, parsing, compiling, and rendering functions.
- Much faster