Created commands (#18)

* Created `Runner` command

* Remove unnesessary temp variable

* Created `AutoRunner` command

`AutoRunner` command executes the `Runner` command every time a file is saved.

* Updated formatting

* Fix descriptions

* Created `AutoRunnerStop` command

`AutoRunnerStop` - stops `AutoRunner` and closes its window

* Automatic formatting

* Added command description to readme

* Update table

Added lua command column to "Commands" table.

* Update function description

Specified that `autorun` can accept buffer number as an argument.

* Fix `autorun` function

When saving a file, now function now checks if `_terminal_windows` exists and call the last shell command, otherwise it calls the vim command.

* Moved `user_commands`

`user_commands` now are created when `setup()` is called.

* Fix README

Removed the line stating that the `setup()` function in optional.

lua/runner/handlers/utils.lua

@@ -5,6 +5,7 @@ local config = require('runner.config')
 M._terminal_buffer = nil
 M._terminal_window = nil
 M._last_command = nil
+M._last_handler = nil
 
 M.create_buffer = function()
   if M._terminal_buffer then