Some fixes and additions

* ADDED: The option to set a temp directory for the ebook parts
* ADDED: The option to set a file path to kindlegen instead of requiring to be in PATH for people with permission issues or wanting to keep it in their repo.
* ADDED: Custom date for the issue.
* FIXED: Date of the issue displayed on the Kindle.
* FIXED: Path resolution.
* UPDATED: README with all of the above.
* CHANGED: Underscores instead of truncated spaces in title/filename.