vis-lua: add vis:command_register to map a Lua function to a :-command

The following registers `:foo` as a command which prints a few things to
stdout:

 vis:command_register("foo", function(argv, force, win, cursor, range)
	 for i,arg in ipairs(argv) do
		 print(i..": "..arg)
	 end
	 print("was command forced with ! "..(force and "yes" or "no"))
	 print(win.file.name)
	 print(cursor.pos)
	 print(range ~= nil and ('['..range.start..', '..range.finish..']') or "invalid range")
	 return true;
 end)

1 files changed, 1 insertions, 0 deletions

diff --git a/README.md b/README.md
index 96bbec5..1abd8b2 100644
--- a/README.md
+++ b/README.md
@@ -594,6 +594,7 @@ At this time there exists no API stability guarantees.
    - `textobject(id)` select/execute a text object
    - `motion_register(function)` register a Lua function as a motion, returns associated `id` or `-1`
    - `motion(id)` select/execute a motion
+   - `command_register(name, function(argv, force, win, cursor, range))` hook up a Lua function to `:name` command
    - `map(mode, key, function)` map a Lua function to `key` in `mode`
  - `file`
    - `content(pos, len)`