Generate requirements.txt file for multi path

My project has an src directory and a tests directory, I want to generate a requirements.txt file of them like this:
```
pipreqs src tests
```
But it doesn't seem to support the pass multi directory at once right now.

Start at the pipreqs command entry point and inspect how its current path argument is parsed. Try the requested `pipreqs src tests` invocation against a project with both directories, then make completion mean that imports from both paths are represented in the generated requirements.txt file.

Written by the indexing model from the issue text.