Support keyword arguments in all functions and filters with optional parameters

[TheWebTech]

It's easier to explain this by showing a use-case first.

Developer wants to use a filter that has 1 required param, and has 2 optional params |filter(param1, param2, "param3") The developer only wants to specify param 1 and param 3 |filter(param1, "param3") This method would cause param 3 to be interpreted as param 2 and not work how the developer might expect.

Theoretically the developer could do this instead: |filter(param1,undefined,"param3") This has some issues, such as defining something to be undefined, little weird. It also means a ton of typing if you only need 1 optional argument out of say 5.

In at least one case with our filters a developer can now do this: |filter(param1, param3="param3") One advantage of this is if a function ever undergoes a breaking change such as to the ordering of parameters, it won't be an issue for a developer that built this way.

We don't believe this is currently set up to work with all functions and filters with optional parameters. That's the goal of this issue.

[liamrharwood]

related: #500

[michaelpro1]

If you are interested in changing specific filters to support key-value arguments, take a look at IndentFilter that uses a new AbstractFilter to convert array parameters to map. It also includes some basic parameter conversions, and is dependent on Jinjavadoc of the filter

[mgns]

Is there anything speaking against applying the AbstractFilter to all filters? If not, then it seems a straightforward solution to apply the same concept to the remaining filters. Having support for named arguments would bring Jinjava "closer" to the official Jinja documentation.