Add custom directive to be consumed by external scripts

[skorulis-ap]

This adds a // @knit custom("...") directive which knit simply passes through into the JSON output. This allows external apps to define custom tags that can be consumed by scripts.
The current planned usage is to have something like // @knit custom("skip_validation") which will be consumed by an external validate command to provide an escape hatch for edge cases.

I'm not particularly wedded to the name custom so if anyone has a suggestion please let me know.

[bradfol]

What do you think about including the code for the validation logic within Knit as well? That would make it more of a first-class command than a custom passthrough

[skorulis-ap]

What do you think about including the code for the validation logic within Knit as well? That would make it more of a first-class command than a custom passthrough

I've been thinking this through and for the first version of validation I want to treat it as a problem specific to a single codebase and then later look at how it can be generalised.