Added option to choose UltiSnip expand function

[exdis]

By default plugin uses UltiSnips#ExpandSnippet(). And if g:CleverTabUltiSnipsExpandOrJump setted, UltiSnips#ExpandSnippetOrJump. This helps with autocomplete inside UltiSnips tabstops.

[chaucerbao]

Good idea. I prefer UltiSnips#ExpandSnippet() over UltiSnips#ExpandSnippetOrJump(), because after I expand a snippet, I can't tell if my next <tab> press will jump to the next UltiSnip marker, or run a CleverTab expansion.

This simplifies things and makes the behavior predictable.