Automatically map resources to dirs in work dir

[bradfordboyle]

For Concourse tasks that have several input/output resources, it can be tedious to write out explicit resource mappings via -i or -o when using piper from the command line (versus inside of a bash script). This commit adds functionality to look for input and output resources in the current working directory, matching on the resource name.

[bradfordboyle]

I haven't added tests for this feature yet, but I wanted to get the idea in front of people to see if its even worth adding. Would love feedback on how to make this testable (assuming the feature is wanted/needed).