Added support for overriding start and end strings

[robisaac]

The code is backward compatible because the opts are optional, so it's not a breaking change.

[mattrobenolt]

Might I suggest an API something like:

$ jinja2 --env-opt variable_end_string=#} --env-opt variable_start_string={#

This way we don't need a flag for every single option and just directly pass those through?

[rob-spoor]

I've created a pull request for the change you suggested: #80. It supports boolean, int and string options.