Support Jinja2 filters

[shawnmjones]

Raintale templates are based on Jinja2 templates. With the addition of preferences, Raintale templates are no longer Jinja2 compatible, but Jinja2 filters would still be useful to users.

The differences:

• Preferences instruct Raintale how to generate the value for a variable. Example: choosing the 3rd best sentence from the document.
• Filters instruct Jinja2 how to alter the value of a variable after creation. Example: converting the text to uppercase.

Raintale should still support Jinja2 filters.

Preferences are handled like so: {{ variable|prefer key=value,key2=value2 }}

Jinja2 filters could be added to the mix like so: {{ variable|prefer key=value,key2=value2|filter1|filter2(arg1, arg2)|filter3 }}

[shawnmjones]

This is currently working in the branch at cf19df107d894da9298aa50c728276d61e784bbf.

[shawnmjones]

This has been done for a while.