Load snippets with neosnippet.vim dynamically.

## Problem
The snippet file, that is go.snip, is not loaded with neosnippet.vim when golang source file is opened in new windows.

This is related to [Pull Request #181](https://github.com/fatih/vim-go/pull/181).

## Solution
Load go.snip file explicitly by executing `NeoSnippetSource` whenever golang source file is opened.

## Modification
This pull request consists of modifications as follow:
- Execute `NeoSnippetSource` instead of setting `g:neosnippet#snippets_directory`.
- Avoid checking `g:go_loaded_gosnippets` to load `ftplugin/go/neosnippet.vim`