Add "include" option to specify patterns to look for in the input directory

[prantlf]

If the markdown files are in multiple directories and the common parent directory contains lots of other files and directories, sometimes it is easier to specify what files and directories to include than what patterns to exclude. For example:

dist/
docs/
src/
node_modules/
README.md

And how to build a single PDF from the documentation:

concat-md --include README,docs . | md-to-pdf > docs.pdf