snippets: Some golang snippets not expanding

[nvkalinin]

Steps to reproduce:

1. Install micro and run plugin install snippets command.
2. Open any .go file (e. g. micro main.go) and put the following text (make sure lf is indented with a tab):

package main

func main() {
	lf
}

3. Place cursor right to lf and run the snippetinsert command (or use Alt-S).

Result: it says Unknown snippet "lf"

Expected behavior: lf expands to log.Printf("%s", var)

I looked through the snippets.lua code. It seems that the CursorWord function is problematic. It looks back for a word until it encounters a space (literally: if (r == " ") then) or beginning of the line. But in go they use tabs for indents. So it captures \tlf as a snippet name.

I think it should check for any white space, not just "space".