Use length caching in JS `for` snippet

The previous version of this snippet referenced the object's length on
each iteration of the loop. This adds unnecessary overhead, causing the
loop to be slower. While this will not make much of a difference in many
cases, if the for loop was to be used on a large object or in a tight
loop, it would degrade performance.

Perhaps more importantly, if you were to not cache the length of a live
query, such as DOM queries, you would see significant performance
degradation. If nothing else, this adjustment to the snippet sets things
up for success in more scenarios.
Increase scope of JS `for` snippet placeholder

The `Things.length` part of the JS `for` snippet previously did not
include the `.length` portion. Using some regex replacement, we can
include this bit in the placeholder while still only using the `Things`
bit inside the `for` loop. This should make the snippet slightly more
pleasant to use.
Improve descriptions of JS `for` snippets

More specifically than "faster" and not faster, these snippets count
down and count up, respectively. Adding this information to the
descriptions will help people make the correct decision.
Change JS `for` counting down trigger from for to ford

As suggested by @SirVer[1], this commit changes the trigger of the
JavaScript `for` snippet that counts down from "for" to "ford". Since
"for" is used pretty often, this will prevent people from needing to
select which one they want from the menu each time.

[1]: https://github.com/SirVer/ultisnips/pull/123/files#r8686514
Add "b" options to JS `for` and `ford` snippets

`for` loops usually appear at the beginning of the line. This commit
makes these snippets more context-aware by adding the "b" option that
prevents them from being triggered if they are preceded with something
other than whitespace, as suggested by @SirVer[1].

[1]: https://github.com/SirVer/ultisnips/pull/123/files#r8686514
Simplify JS `for` snippet bodies

As suggested by @SirVer[1], having `Things[i]` be pre-populated in the
body of the `for` loop snippets is not very useful given the merits of
autocompletion. This commit simplifies these bodies by simply using
`${VISUAL}$0`.

[1]: https://github.com/SirVer/ultisnips/pull/123/files#r8686533
Re-order JS `for` snippets

Now that we have separated the two `for` loop snippet triggers by
changing the one that counts down from "for" to "ford", it feels more
and more like "ford" is a variant of "for" so I am moving it after it in
the file. Also, alphabetical.
Merge pull request #123 from lencioni/improve-js-for-snippet

Javascript: Improve JavaScript `for` snippets