allure commandline: support building report when results and categories.json are in different folders

**Is your feature request related to a problem? Please describe.**

Currently `bin/allure` requires that both `simple-result.json` and `categories.json` are in the same folder.
If I put the files into different folders and pass both of them to `bin/allure serve`, then the report does not pick up the categories.

The problem of having different data in the same folder looks like "overlapping outputs" to the build systems.
Overlapping outputs incur extra burden on builds systems (e.g. https://docs.gradle.org/current/userguide/build_cache_concepts.html#concepts_overlapping_outputs)

**Describe the solution you'd like**

I would like to provide an explicit pair of "results" and "categories", so allure could build the report accordingly.

For instance: `./allure serve --results results1 --categories path/to/categories.json --results results2 --categories another/categories.json`
An alternative option is to prepare the description in a standalone JSON file and pass it to `./allure serve --input inputs.json`

**Describe alternatives you've considered**

An alternative would be producing files into different folders and create yet another folder that combines all the files right before calling `allure`. However, it would require extra file copy operation.

Start by reading bin/allure and tracing how the serve command discovers results and categories.json. Reproduce the separate-folder case from the issue, then define and verify an input form that associates each results folder with its categories file and includes all categories in the generated report.

Written by the indexing model from the issue text.