Add a !deck command to decode deckstrings

[jleclanche]

Example usage:

> !deck AAECAZ8FBvQFucECg8cC1uUCteYCt+cCDKcF9QXZB7EI2a4Cu68C/68CuMcC2ccC48sC+NICieYCAA==

Reply (surrounded by triple-backticks):

# Class: Paladin
# Format: Standard
# Year of the Mammoth
#
# 2x (1) Argent Squire
# 2x (1) Lost in the Jungle
# 2x (1) Righteous Protector
# 1x (2) Drygulch Jailor
# 1x (2) Equality
# 2x (2) Dire Wolf Alpha
# 2x (2) Knife Juggler
# 2x (3) Divine Favor
# 1x (3) Unidentified Maul
# 2x (3) Steward of Darkshire
# 2x (3) Rallying Blade
# 2x (4) Call to Arms
# 2x (4) Lightfused Stegodon
# 2x (5) Stand Against Darkness
# 1x (5) Level Up!
# 1x (6) Sunkeeper Tarim
# 2x (6) Crystal Lion
# 1x (7) Vinecleaver
#
AAECAZ8FBvQFucECg8cC1uUCteYCt+cCDKcF9QXZB7EI2a4Cu68C/68CuMcC2ccC48sC+NICieYCAA==
# Find more decks on https://hsreplay.net/decks/

[jleclanche]

Decode the deckstrings with the hearthstone.deckstrings module.

https://github.com/HearthSim/python-hearthstone/blob/master/hearthstone/deckstrings.py

[troggnostupidhs]

Added in this PR #8