More snippets for go-mode. Removed some verbosity in text of old snippets.

Just added some useful snippets for Go. Also changed argument names in old snippets for clarity.
Nice trick with variable. I tried to work with this variant of `if` for a day but found it uncomfortable. Let me explain. Often we have multiline body in the `if` statement and this body may include inner `if`, `else`, `for` and other constructs with repeatable use of yasnippet. Then we break yasnippet execution for outer `if` and left outer `else` expanded. It is not convenient I think. For me better to use separate snippets for `if` and `else` and call `else` excplicitly. But I renamed `else` to `el`. Maybe rename `switch` → sw and `select` → sel too?