Use Recast for AST modification in custom.js

The https://github.com/benjamn/recast package is similar in spirit to `falafel`, but it has more convenient tools for checking AST node types and creating new AST nodes, which helps simplify the big `if`-statement in `custom.js` and also makes it easier to create the replacement `Literal` nodes.

If that's not something you particularly care about, then feel free to close this pull request. Just thought I'd make the suggestion!