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
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todo
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fix #100
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
add gprof for debug
todo
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.
Merge branch 'master' of github.com:coolwanglu/pdf2htmlEX
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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.