move ->JavaScript to compile, fix "-_" issue

[sritchie]

This is a combo.

feature

Right now, the ->JavaScript function lives in render; it takes an expression and creates a function out of it by extracting variables.

The problem here is that we don't know the variable order! A better approach is for the user to pass an explicit function to be "compiled" into js.

bug

the bug is that the js renderer will pass on kebab-cased variables like this-one; that is invalid in JS, and dashes should be converted to underscores.