Add a prepend/append to nth tag feature to prepend.lua

[biki-desu]

^^ what the title says

this should be easy

[biki-desu]

The logic for this feature has been added, now we just need to integrate it into the gui.

[biki-desu]

HOW TO IMPLEMENT THIS IN THE GUI???
edit: current gui: http://i.imgur.com/JQYz1SH.png

• 4 extra buttons: prepend/append nth tag, increment, decrement, displaying the count in the P/A buttons

or

• 2 extra buttons (prepend/append nth tag), add a intedit control for c_nth and get rid of P/A first/last tag (get rid of 4 buttons)

or

• 2 extra buttons (prepend/append nth tag), add increment&decrement buttons for c_nth, get rid of P/A first/last tag (get rid of 4 buttons) and display the count in the actual P/A buttons