Added support for altcoins

[nbarraille]

Replace hardcoded pchMessageStart and address version bytes to support a bunch of altcoins.

Haven't tested all features but extracting addresses from transaction works

[pep8speaks]

Hello @nbarraille! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file blockchain_parser/__init__.py:

Line 17:80: E501 line too long (83 > 79 characters)

• In the file blockchain_parser/address.py:

Line 34:80: E501 line too long (85 > 79 characters)

• In the file blockchain_parser/output.py:

Line 62:80: E501 line too long (84 > 79 characters)
Line 66:80: E501 line too long (84 > 79 characters)
Line 71:80: E501 line too long (86 > 79 characters)