Task widget

A widget used to play with taskwarrior (`pacman -S task`) tasks. It provides naughty notification for pending tasks, prompt to add new task, and prompt to mark tasks done/deleted.

Here's rc.lua example:

```lua
-- Textclock
clockicon = wibox.widget.imagebox(beautiful.widget_clock)
mytextclock = awful.widget.textclock(markup("#7788af", "%A %d %B ") .. markup("#343639", ">") .. markup("#de5e1e", " %H:%M "))

-- Calendar
lain.widgets.calendar:attach(mytextclock, { font_size = 10 })

-- Task
taskwidget = lain.widgets.task:attach(clockicon, { font_size = 10 })

globalkeys = awful.util.table.join(
    ...
    awful.key({ modkey,           }, "t",      lain.widgets.task.prompt_add),
    awful.key({ modkey, "Shift"   }, "t",      lain.widgets.task.prompt),
    awful.key({ modkey, altkey    }, "t",      lain.widgets.task.show),
    ...
)
```