Implement missing built-ins

[tdammers]

Ginger does not currently implement all the built-in functions, filters and tests that jinja2 defines.

For reference:

• Jinja2 Built-in Filters

• Jinja2 Built-in Tests

• Jinja2 Built-in Global Functions

• Ginger Filters / Functions (as documented)

• Default global context definition in the Ginger source

Note that in Ginger, these things are all just syntax sugar for function calls, while Jinja2 has separate namespaces for each of them - we are not looking to change this, just implementing missing things.