Display source of error in message

[sebastienros]

Fixes #449

[sebastienros]

Result:

'=' was expected after the identifier of 'assign' at (2:14)
Source:
  {% assign a 'b' %}

And it keeps the spaces at the beginning of the line such that the column number is correct.

[sebastienros]

@deanebarker works for you?

[deanebarker]

@sebastienros Yes, it appears to work perfectly. Thank you.