Support piped input for generating documentation

[robross0606]

For automation purposes, it would ben useful to be able to pipe IDL input into the tool versus having to write out a temporary file and then pointing the tool to that file (IDL, etc.).

For example:

npm run schema:out | npx graphdoc -p -o ./doc/graphdoc

[robross0606]

This may already be possible somehow, but the documentation is extremely vague and looks slightly corrupt and possibly outdated in the README:

Generate documentation from for the "modularized schema" of graphql-tools

graphdoc -s ./schema.js -o ./doc/schema ./schema.graphql must be able to be interpreted with graphql-js/utilities#buildSchema

It is very unclear what is expected out of ./schema.js.