Explorer API: add support for `address_info` endpoint

[parodyBit]

example:
https://witnet.network/api/address_info?address=wit1drcpu0xc2akfcqn8r69vw70pj8fzjhjypdcfsq

returns:
{"wit1drcpu0xc2akfcqn8r69vw70pj8fzjhjypdcfsq":{"address":"wit1drcpu0xc2akfcqn8r69vw70pj8fzjhjypdcfsq","label":"","active":1890302,"block":65,"mint":65,"value_transfer":90,"data_request":4272,"commit":1802,"reveal":1802,"tally":2987}}

Adding this endpoint will provide data to show about a node when it is imported into myWitWallet