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
I don't like the format that these errors are displayed in. Perhaps `Liquid error (line 3): stuffstuffstuff`?

Also it seems bad that I couldn't immediately distinguish which part of that was syntax error and which was original text. I know that currently Liquid does this but might it be better to have errors in some kind of brackets to separate them from surrounding content. Might be a job for another PR. Something like:
```
[Liquid error (line 3): standard error] will raise a standard error.
```