Support absolute paths for schema location

[ColinEberhardt]

Splitting this out from here, as it's a separate issue: #171 (comment)

[ColinEberhardt]

This might be a Windows issue, the following works on Mac:

% node src/index.js forge \
                /Users/colineberhardt/temp/openapi.json \
                https://github.com/ScottLogic/openapi-forge-javascript.git \
                -o api