Let users supply their own crypto signatures files

Related to #220. Solve this only after #220.

Related to #221, #222, #223, #224, #225. Solve them together or close one after another. Share design decisions - uniform solutions.

Let users supply their own crypto signatures files.

Probably refactor the whole system in the process:
- Refactor the directory structure. Something like `share/retdec/support/crypto-patterns/`.
- Let users specify their own pattern file(s) or whole directory(s) through `retdec-decompiler.sh` script parameter. Specified files can be both text YARA and compiled YARA rules.
- Let users add their files to the user support directory (e.g. something like `share/retdec/user-support/crypto-patterns`). These will be always used, without user needing to specify them via script parameters.
- Write a how-to wiki page. Encourage users to contribute their signatures to upstream.

Read issue #220 first, then review the related issues and shared design decisions. Inspect retdec-decompiler.sh and the proposed share/retdec/support/crypto-patterns and share/retdec/user-support/crypto-patterns locations. Done means user-supplied text or compiled signature files are loaded, user-support files are always used, and a how-to wiki page explains contribution.

Written by the indexing model from the issue text.