Added ability to explicitly specify page name format when splitting
output across multiple pages. Can include page number position
in the output file name.

Examples:
pdf2htmlEX --split-pages 1 foo.pdf bar%d.html
pdf2htmlEX --split-pages 1 foo.pdf bar%02d.page
Merge remote-tracking branch 'upstream/master'
added input sanitation for split page generation when accepting a custom outfile file name format. Added unit tests for various file name generation scenarios.
updated split page filename formatting to not rely on regex to be compatible with older compilers. added several test cases to account for new implementation. updated documenation to more accurately reflect how split page filenames are generated.
Merge remote-tracking branch 'upstream/master'
fixed missing include causing ci build to fail; cleaned up warnings about signed/unsigned comparison; cleaned up local string declarations
updated sanitize filename code to use size_t instead of raw number types
combined the sanitize and validate funcitons into a single function; limited the format characters supported to avoid validation complexity; updated documentation; feature implemented by Ryan Morlok (ryan.morlok@morlok.com) on behalf of Docalytics (http://www.docalytics.com/)
Merge pull request #98 from rmorlok/master

Added ability to customize page name format for split rendering