Ensure backwards compatibility with older eglot versions

- There are issues for old eglot versions installed via packages (ELPA or MELPA or version control)
- There are issues with older eglot versions bundled with emacs

Fix #47

Make the auto-configuration settings usable on NixOS

- Allow optionally a custom config folder
- Do not modify in-place eglot-java-eclipse-jdt-args
- Find the relevant jar via a recursive search from the root JDT LS folder

Fix #46

Fix org-mode markup