Implement support for processing arrays

[SeriousBug]

There should be a way to specify a separator in a template, which will split a value by that separator then feed each subvalue through the created pipeline. Then the template writer should have to specify a joiner to join the processed subvalues. Maybe something like:

{{ variable | separate , | capitalize | join , }}

In this vein, developers should be able to add their own splitters and joiners. You could add a SUM or AVG or MEDIAN joiner for example.