Add SaveAs action and command

Fixes #340

You can bind the action `SaveAs` and if you provide an argument to
the `save` command it will save as. For example `> save test.txt`.

cmd/micro/actions.go

@@ -754,6 +754,21 @@ func (v *View) Save(usePlugin bool) bool {
 	return false
 }
 
+// SaveAs saves the buffer to disk with the given name
+func (v *View) SaveAs(usePlugin bool) bool {
+	filename, canceled := messenger.Prompt("Filename: ", "Save", NoCompletion)
+	if !canceled {
+		// the filename might or might not be quoted, so unquote first then join the strings.
+		filename = strings.Join(SplitCommandArgs(filename), " ")
+		v.Buf.Path = filename
+		v.Buf.Name = filename
+
+		v.Save(true)
+	}
+
+	return false
+}
+
 // Find opens a prompt and searches forward for the input
 func (v *View) Find(usePlugin bool) bool {
 	if usePlugin && !PreActionCall("Find", v) {

cmd/micro/command.go

@@ -237,8 +237,12 @@ func Quit(args []string) {
 
 // Save saves the buffer in the main view
 func Save(args []string) {
+	if len(args) == 0 {
 		// Save the main view
 		CurView().Save(true)
+	} else {
+		CurView().Buf.SaveAs(args[0])
+	}
 }
 
 // Replace runs search and replace

runtime/help/commands.md

@@ -5,7 +5,8 @@ Here are the possible commands that you can use.
 
 * `quit`: Quits micro.
 
-* `save`: Saves the current buffer.
+* `save filename?`: Saves the current buffer. If the filename is provided it will
+   'save as' the filename.
 
 * `replace "search" "value" flags`: This will replace `search` with `value`. 
    The `flags` are optional.

runtime/help/keybindings.md

@@ -143,6 +143,7 @@ Delete
 Center
 InsertTab
 Save
+SaveAs
 Find
 FindNext
 FindPrevious