Fix #190 - Unknown function: uniq

[guyru]

Define a custom variant of uniq() which offers interoperability for older Vim versions (< 7.4.218).

[lervag]

Agreed to comment by @Oberon00, will merge when that change is done.

[guyru]

I've made the change @Oberon00 suggested (although in the case of single-element lists the logic was fine - the for would have been skipped, returning a copy of the original list).

[lervag]

Still, the change make things more clear and was a (minor) improvement. In any case, thanks! :)