Some improvements to snippets code and to syntax highlighting

[s0ber]

@isRuslan Hi there! First of all, thanks for your package! It is very handy and useful. I've played with it a little bit, and found some small things, which can be improved. So, there are my changes.

First of all, I've decided to use single quotes everywhere, due to AirBnb codestyle (which is commonly adopted by js community):
https://github.com/airbnb/javascript#6.1

I also removed snippets indenting, which was there by default.

Also, your syntax highlighting is great! But it doesn't solve indentation problems. So, whenever you press "enter", vim cursor will likely be in the beginning of the string. And to solve this, I just copypasted vim-javascript indentations rules. It works ok so far.

Another small issue, that was hurting me — => snippet was not working in the middle of the string. I've fixed this only for UltiSnips snippet. (I've tried to use both of them, UltiSnips and Snipmate, and found out, that UltiSnips is more stable and simple.)

And the last thing — there is small improvement to class declaration.

I hope those changes are useful.

[s0ber]

@isRuslan Maybe we can add indentation rules as a submodule from vim-javascript.

[isRuslan]

Wow, what are nice and useful changes, thanks @s0ber 👍
We need indentation support here, to not be depend on other javascript-like vim-plugins, that's true.
So vim-javascript/indent be copy pasted here for a while, then I'll refresh it.
Thank for PR, this will be published as v2.1

[s0ber]

@isRuslan Thank you too! This is the best es6 plugin so far! :)