Add support for user-created commands

Plugins can now create their own commands using the `MakeCommand`
function. Plugins can also now create their own keybindings with the
`BindKey` function. See the go plugin for an example of `MakeCommand`.

runtime/help/help.md

@@ -152,6 +152,9 @@ Here are the possible commands that you can use.
 * `run sh-command`: runs the given shell command in the background. The 
    command's output will be displayed in one line when it finishes running.
 
+* `bind key action`: creates a keybinding from key to action. See the sections on
+   keybindings above for more info about what keys and actions are available.
+
 ### Options
 
 Micro stores all of the user configuration in its configuration directory.

runtime/plugins/go/go.lua

@@ -5,6 +5,9 @@ if GetOption("gofmt") == nil then
     AddOption("gofmt", true)
 end
 
+MakeCommand("goimports", "go_goimports")
+MakeCommand("gofmt", "go_gofmt")
+
 function go_onSave()
     if views[mainView+1].Buf.FileType == "Go" then
         if GetOption("goimports") then
@@ -12,21 +15,25 @@ function go_onSave()
         elseif GetOption("gofmt") then
             go_gofmt()
         end
-
-        views[mainView+1]:ReOpen()
     end
 end
 
 function go_gofmt()
+    views[mainView+1]:Save()
     local handle = io.popen("gofmt -w " .. views[mainView+1].Buf.Path)
     local result = handle:read("*a")
     handle:close()
+
+    views[mainView+1]:ReOpen()
 end
 
 function go_goimports()
+    views[mainView+1]:Save()
     local handle = io.popen("goimports -w " .. views[mainView+1].Buf.Path)
     local result = go_split(handle:read("*a"), ":")
     handle:close()
+
+    views[mainView+1]:ReOpen()
 end
 
 function go_split(str, sep)