J2gpp

- [Fix] Preserve indent in align filter block

- [Feature] Write and append filters to export content of block to other file
- [Feature] Indentation filters for block
- [Feature] Align filter for block

- [Doc] Explain the command line options missing from documentation

- [Feature] Ability to include a variables file from another using the `__j2gpp_include__` attribute
- [Feature] Command line argument to provide a Python function to process the variables dictionaries after loading from file
- [Feature] Warn when an attribute is not a valid identifier and may not be accessible from the templates
- [Feature] Option to disable checking valid identifiers
- [Feature] Option to automatically fix invalid identifiers with underscores

- [Feature] Additional built-in Jinja2 filters
- [Feature] Custom filters by providing python file in command line argument
- [Feature] Custom tests by providing python file in command line argument
- [Debug] Option to display all available variables at the top of the renderd files

- [Feature] Command line option to warn when overwriting files
- [Feature] Command line option to prevent overwriting files
- [Feature] Command line option to overwrite the entire output directory
- [Fix] Actually render templates when also copying non-templates
- [Usability] Check incoherence of command line arguments