Added pre-build and post-build hooks

[Nealium]

Added hooks feature. Allows users to tac on any extra project specific build functionality, really the sky is the limit. The intent is to use hooks when the pyinstaller exe is the end goal and it won't be bundled with a wheel

Also: Added the EEXIST error code import that was missing in extended_includes. (stage hunk mishap 😞)

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Edit

I didn't think my original README examples were very good, especially running tests, So I've added more applicable hook examples:

• pre-hook:
  • Building local docs with mkdocs
• post-hook:
  • Adding version to output my-project/ -> my-project_1.0.0/
  • Removing mkdocs's site and pyinstaller's build directories