Recognize null as a literal in the lexer

[hniksic]

Proposed fix for #140.

In lib/Lexer.js, the _createToken method handles true and false as boolean literals:

} else if (element === 'true' || element === 'false') {
  token.value = element === 'true'
}

But there's no corresponding handling for null. null is tokenized as an identifier and looked up in the context, returning undefined.

This explains the strange behavior observed in #140:

1. [null] parses as [context.null] which evaluates to [undefined]
2. x in [null] with {x: null} becomes null in [undefined], and evaluates to false
3. x in [null] with {} or {x: undefined} becomes undefined in [undefined], and evaluates to true`

If we handle null in _createToken:

} else if (element === 'true' || element === 'false') {
  token.value = element === 'true'
} else if (element === 'null') {
  token.value = null
}

...program from #140 works as expected, and JEXL's existing tests pass.

Note: PR #134 works around this in arithmetic operators by treating both null and undefined as "nullable", but that doesn't help operators like in that need to distinguish between the two.