Macros don't seem to be supported in plugin (e.g. for use with option --suppressions-list )

### Environment

* Operating System (e.g. Ubuntu 16.04 x64): MS Windows 10 Pro (Version 10.0.18363 Build 18363)
* IDE Version (e.g. CLion 2016.3.2): CLion 2019.3.4
* Cppcheck executable version (`cppcheck --version`): 2.1
* Cppcheck plugin version: 1.4.2
* Exact strings used in cppcheck plugin options:
* **cppcheck path**: C:\\Program Files\\Cppcheck\\cppcheck.exe
* **cppcheck options**: --enable=warning,performance,portability,style --language=c++ --suppressions-list=$ProjectFileDir$\\.cppcheck-suppressions

### Expected behaviour

It would be nice if we could have a suppressions-list per project, e.g. using $ProjectFileDir$ as shown above.

### Actual behaviour

Only a single suppressions-file seems to be supported currently. We need to point to this file with an absolute file path.

Attempting to use macro $ProjectFileDir$ as shown above results in the following to appear in an error dialog:
ExecutionException: Cppcheck Error : Exit Code - 1 : "C:\Program Files\Cppcheck\cppcheck.exe" "--template=[{file}:{line}]: ({severity}) {id}: {message}" -IC:/Projects/src/work/\/src --enable=warning,performance,portability,style --language=c++ --suppressions-list=$ProjectFileDir$\\.cppcheck-suppressions C:\Users\DANIEL~1\AppData\Local\Temp\___xxxxx_processing_client.cc

### Steps to reproduce the behaviour

Just configure the **cppcheck options** as shown above.

Any suggestions? Is there a workaround for this issue that would allow us to set up a suppressions file per project?