Idea: Allow tool/path/.shed.yml as alternative to tool/path/

e.g. Would it be nice to be able to do this:

```
$ planemo shed_lint --report_level warn --tools --fail_level error datatypes/mira_datatypes/.shed.yml
Usage: planemo shed_lint [OPTIONS] PROJECT

Error: Invalid value for "paths": Directory "datatypes/mira_datatypes/.shed.yml" is a file.
```

and have it behave like:

```
$ planemo shed_lint --report_level warn --tools --fail_level error datatypes/mira_datatypes/
```

I'm playing with some scripting ideas, and detecting tools as directories containing a `.shed.yml` file.