Load snippets with neosnippet.vim dynamically for .go file opened in new window.

The go.snip is not loaded when .go files are opened in the new window of Vim.
A variable g:neosnippet#snippets_directory seems to be checked on initialization of neosnippet.vim.

This commit makes Vim load go.snip dynamically when a .go file is opened in the new window.