[md2orchestra] read multiple input files for reusable elements

[donmendelson]

Messages, codesets, components, and fields may be reused across multiple service offerings. It would be desirable to keep reusable elements in their own files and include them in rules of engagement. Therefore, md2orchestra should be capable of reading multiple markdown files to compose an Orchestra file.

[donmendelson]

One implication of this enhancement is that all inputs must be processed before determining which elements are missing and must be retrieved from a reference file.