Add support for starting xml file parsing at some percentage of the file.

[clrogers2]

Currently the stackExchange parser will start at the beginning on the xml file. This poses a problem for large XML files especially when the desired content or Stack Exchange tag is relatively new (occurring at or near the end of the file.)

Provide a way for the user to input a float value between 0 and 1 to specify approximately where in the file to start parsing.