Properly insert snippet for textEdit.

[tigersoldier]

Remember the start and end positions when inserting a completion candidate with textEdit populated. Use those positions to expand yassnippets. insertText is handled in a similar fasion.

To avoid destory previous snippet field, try to restore the pre-completion state by removing text inserted by company first. If company does more than insertion, restore the whole line as how it's done originally.

Fixes #33