FindNextSubmatch: return submatches when searching

[matthias314]

This PR complements FindNext by FindNextSubmatch. In addition to the start and end of the match as a whole this function returns start and end of each submatch, analogous to Regexp.FindSubmatchIndex. I think this is very useful for certain non-trivial search operations, for example for finding the current environment of some position in a LaTeX file. One could also think of other search functions, for example in the spirit of Regexp.FindAllSubmatchIndex. In any case, this PR is at least a start.

This is the first time I work with Go (and Lua). So please check that everything is OK. I've played around with the new function, and it seems to work well. The only issue I have found is possibly due to the way the Go-Lua interface handles slices as opposed to arrays. When accessing a slice element from Lua, I seem to get a pointer instead of a struct. For example, the Lua function

function f3(bp)
    local buf = bp.Buf
    match, found = buf:FindNextSubmatch("a(x|y)b", buf:Start(), buf:End(), buf:Start(), true)
    if found then
    	local cur = bp.Cursor
    	cur:SetSelectionStart(buffer.Loc(match[1].X, match[1].Y)) -- works
     	cur:SetSelectionEnd(match[2]) -- errors
    end
end

gives

bad argument #2 to SetSelectionEnd (cannot use &{3 2} (type *buffer.Loc) as type buffer.Loc)

Is there a way to avoid this?