emacs-company: Support location

`company-mode` has feature to display the location of the selected candidate by using `company-show-location`. This commit enable `company-go` to display the location of candidate. Here is how it looks:

![140213-0001](https://f.cloud.github.com/assets/3079551/2154592/91396196-9437-11e3-9fff-7d2760dedcec.png)