Question: Is there a tool like the subject for Solidity. Let's say I have a contract with import and that import has three imports etc. I'd like to combine all of them into one file and play with it in Remix, for example.

I know how to make it manually.

Answer: 
I'd also suggest you to try this - <http://remix.readthedocs.io/en/latest/tutorial_remixd_filesystem.html>

This feature is currently an alpha version but it may help you.
just for the information. I created a plugin for `VSCode` so you can use it instead of `sol-merger` because it is based on it under the hood.

Here is the link for plugin: [Solidity Contracts Merger](https://marketplace.visualstudio.com/items?itemName=RyuuGan.sol-merger-vscode). 

Hope you will like it.