Add support for bidder alias

[webmarcus]

Would be good to have support for bidder aliases.

[dshore]

@webmarcus something like this in the config yml?

# Only applicable for send all bids (i.e not --single-order)
#
# bidder_aliases:
#   <bidder_code_1>: <new_alias_1>
#   <bidder_code_2>: <new_alias_2>

[webmarcus]

@dshore, as long as it works to achieve the result following the docs, thanks! :)