Allow the --abi flag to be given - to indicate that it should read the
ABI information from standard input. It expects to read the solc output
with the --combined-json flag providing bin, abi, userdoc, devdoc, and
metadata, and works very similarly to the internal invocation of solc,
except it allows external invocation of solc.

This facilitates integration with more complex solc invocations, such
as invocations that require path remapping or --allow-paths tweaks.

Simple usage example:

    solc --combined-json bin,abi,userdoc,devdoc,metadata *.sol | abigen --abi -