Fix thousands of stat calls

I've noticed, that opening files and expanding snippets is very slow on my oldy Atom netbook. After digging around I've come up to running `strace` on vim and after typing `:e lala.go`:

```
$ sudo strace -p `pidof vim` -ttf -o strace.log
$ grep stat ~/strace.log -c
12040
```

Number can greatly depends on amount of plugins one have installed, because every plugin (when using plugin manager) will add it's own path to `&runtimepath`.

I understand concern about looking snippets in the &runtimepath, but most usable way is to have own collection of snippets laying in the single directory.

Proposed patch will change code in that way so UltiSnips will not look for every &runtimepath directory if `g:UltiSnipsSnippetsDir` contains only one absolute path.

Also, patch introduces `g:UltiSnipsDisableSnipMate` option to disable looking for snipmate snippets in `&runtimepath`. I use only UltiSnips syntax and I guess many users so; it's useless to search for snipmate snippets every time. However, value of `g:UltiSnipsDisableSnipMate` is equals to `"0"` by default.

After patch applied:
```
$ grep stat ~/strace.log -c
1256
```

Possible fix for #384.
Completely agree with that.