Improve /potion command

[Andre601]

The /potion command of EssentialsX needs some tweaks and changes to improve it.

Issues and limitations I found so far are as follows:

• /potion clear gives no feedback on vanilla potions
• /potion apply gives no feedback on vanilla potions (Also lacking any tab completion?)
• No option to alter potion colour and colour changes with /potion effect:<effect> amplifier:<number> duration:<number>