Escape RST

[janbrohl]

Add support for escaping RST for security and intuitive behaviour.

• Either escape per default - then ideally something equivalent to autoescaping for RST. Propably writing a Jinja extension is required.
• Or escape on demand via filter.

[janbrohl]

How escaping works depends on context so filtering is the better option.

[janbrohl]

Adding a filter requires modifying an environment so this requires #42 .

[janbrohl]

61 adds the escaping logic as functions. Filters woulld better looking.