Optional line numbers for liquid errors

Adds a line_number flag to Liquid::Template.parse(), which will add line numbers to errors.

Note that this does *not* add line numbers to warnings for the strict parser in warn mode. Couldn't figure out yet what the best way is to do that.

@trishume, thoughts?

Part of https://github.com/Shopify/liquid/issues/245
Is this a good name given that there will not always be a line number?