Fix multi argument indent when using noexpandtab; make it optional

[idbrii]

Cleaned up version of suokko's fix from #2 to more closely match existing code style and naming.

We must \t as &shiftwidth instead of a single char. Can't use
GetStringIndent because it stops when it hits non whitespace and making
it support both cases is awkward.

With noet sw=8 ts=8 old behaviour:

f = function(a,
	     b)
	print(a)
	g = function(a,
	      b)
	end
end

New behaviour fixes indent for g:

f = function(a,
	     b)
	print(a)
	g = function(a,
		     b)
	end
end

---

Also add g:lua_indent_align_params. let g:lua_indent_align_params = 0 for single indent for function params:

  f = function(a,
      b)
      print(a)
      g = function(a,
          b)
      end
  end
  g = function(a,
      b)
  end

  create(10,
      20,
      30,
      get(
          1,
          2),
      val())

Trying to evaluate indentation in this plugin vs tbastos/vim-lua, so I thought I'd contribute some changes I made along the way. In the end, tbastos's version seems like it'll better support chained function calls so that may be a deciding factor for me.