Fix for Issue #83: Leading Zeros

In addition to issue #83, this falls along the same lines as the closed pull request #91.

This provides a command line option --leading_zeros, which when applied to commands extracting individual pages, will pad the page numbers in the file's name. Instead of file_1.png, file_10.png, file_11.png, ... file_2.png, the files will be named file_01.png, file_02.png, ..., file_09.png, file_10.png, file_11.png.