Renamed $string to $source. See #143 and #147

Don't know what to do with the conditional yet. Looked for `loadFile` and `load` as suggested in #143 but didn't find those methods/functions in a quick search of the repo on Github. Will look into it in a bit.
Found the file load function. Closes #143 and #147

Found the file load function here: https://github.com/Masterminds/html5-php/blob/2cad9c173f14eb390f78100dd74ef005f67e1fa5/src/HTML5.php#L66
Merge pull request #148 from sandeepshetty/patch-1

Fixes #143 and #147