This has few advantages:
* `Map` is more performant, and its keys cannot clash with builtin
JavaScript properties (e.g. `toString`).
* TypeScript has better type information about `map.keys()`, whereas
`Object.keys()` always return `string[]`.
Also, move `camelize` inside `getBooleanElements`, as it's only used
there, and unify the 2 `for` loops in `bindBooleanOptions` (iterating
`options` is wrong because it might contain options that doesn't exist
in the page).
relrelb