Add functionality for binding mouse buttons

This commit enables users to bind the mouse buttons (left, middle,
right buttons and the scroll wheel).

The default bindings now include the mouse bindings:

    "MouseWheelUp":   "ScrollUp",
    "MouseWheelDown": "ScrollDown",
    "MouseLeft":      "MousePress",
    "MouseMiddle":    "PastePrimary",

Mouse buttons can now also be bound to normal actions. For example:

    "MouseLeft": "Backspace"

This also means that plugins can access mouse event callbacks in the
standard way ('onAction').

More documentation for this will be coming soon.

Fixes #542

cmd/micro/plugin.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/yuin/gopher-lua"
+	"github.com/zyedidia/tcell"
 	"layeh.com/gopher-luar"
 )
 
@@ -60,6 +61,16 @@ func LuaFunctionBinding(function string) func(*View, bool) bool {
 	}
 }
 
+func LuaFunctionMouseBinding(function string) func(*View, bool, *tcell.EventMouse) bool {
+	return func(v *View, _ bool, e *tcell.EventMouse) bool {
+		_, err := Call(function, e)
+		if err != nil {
+			TermMessage(err)
+		}
+		return false
+	}
+}
+
 func unpack(old []string) []interface{} {
 	new := make([]interface{}, len(old))
 	for i, v := range old {