Allow passing multiple files instead of just one

[gabotechs]

Right now we are limited to pass just 1 entrypoint to the tree and entropy command, which is supposed to be the main executable/library file for a project. Some projects might have multiple entrypoints, either because they ship multiple executables or because they are libraries that do not expose everything from a single file.

This would also solve https://github.com/gabotechs/dep-tree/issues/62.

TODO:

• [x] smarter language inference based on the array of files passed
• [x] recover some root_tests that where commented
• [ ] more testing for when multiple files are passed
• [ ] more testing for when entrypoints cannot be inferred from the graph nodes with indegree 0
• [x] ~consider using a mechanism for passing a massive amount of files~ Not in this PR
• [x] verify that the per-language Display implementation works fine, or consider extending it
• [x] with the new Display implementation colors might look weird, as there is always one common dir