Add ICU prettifier

[samhh]

Closes #129. For now the text content must be supplied as per the example instead of via stdin; a change here could be covered by #110.

$ cat x.json
{
  "x": {
    "message": "{hasTags, boolean, true {{type, select, overLimit {{upperLimit, number}+ best free {formattedListOfTags} photos on Unsplash} belowLimit {{photoTotal, number} best free {formattedListOfTags} photos on Unsplash}}} false {{type, select, overLimit {{upperLimit, number}+ best free photos on Unsplash} belowLimit {{photoTotal, number} best free photos on Unsplash}}}} {another, select, a {{foo}} b {bar}}"
  }
}
$ intlc prettify $(cat x.json | jq -r .x.message)
{hasTags, boolean, 
	true {{type, select, 
		overLimit {{upperLimit, number}+ best free {formattedListOfTags} photos on Unsplash}
		belowLimit {{photoTotal, number} best free {formattedListOfTags} photos on Unsplash}
	}}
	false {{type, select, 
		overLimit {{upperLimit, number}+ best free photos on Unsplash}
		belowLimit {{photoTotal, number} best free photos on Unsplash}
	}}
} {another, select, 
	a {{foo}}
	b {bar}
}

There isn't really a satisfying way to format ICU messages across newlines. In this example the space between the hasTags and another interpolations needs to be preserved exactly as-is. Equally formatting in callbacks would be ambiguous, whereas the newlines and indentation inserted here aren't because it's within the syntactic structure of the interpolations.

I think this is okay because as per the feature request, and the part of the example I borrowed from there, we'll mostly be using this for interpolation-heavy, flattened messages.

In terms of implementation, the bulk of the change is refactoring the ICU compiler to be a little more abstract. Following that the change to support formatting is straightforward and centralised, essentially just using Reader local to increment a count according to how deeply nested into "cases" in the AST we are and inserting formatting accordingly.

[OliverJAsh]

Nice! This will be useful. Should we add this to the docs?