completion: fix insertion point

[Mic92]

The completion point is currently ignored, so it will add too much text, when completing.
A possible algorithm is here: https://github.com/justmao945/vim-clang/blob/master/plugin/clang.vim#L592

How to reproduce:

  #include <string>
  int main() {
->    std::string b = "Hello world!";
#include INSPECTOR
  }
> b.len<TAB><TAB>
> b.lenlength # wrong behavior at the moment
> b.length # that would be the right behavior

the start_position needs to computed correctly.

[Mic92]

fixed