Improve output files creations

[rdeltour]

We could add options to:

• override existing files or not
• clean the entire output directory, or resources sub directories

[amesga]

If the output files are created in a subfolder named after the ePub file, a user wouldn't have to change output dirs for each ePub and could keep the -o switch the same when running a batch of books. Right now, I am manually adding each report to a subfolder before running the next book through.

[rdeltour]

New features in next version:

• check the presence of existing report files, and abort by default
• --force option to override any existing files
• --subdir option to create the report in a sub-directory named after the input EPUB