Execution command as a list instead of string

I use the extension for testing, so I keep a list a list of different compilation commands for c++.
The problem is that I have a lot of options and flags for testing and maintaining as a string is terrible...

The following suggestion has the issue of making the `settings.json` too big, this could be solved with a specific file for the extension.

Suggestion:
Allow to user to set the commands in 3 different ways:
1) current way, a complete string e.g
```json
"c": "cd $dir && clang $fileName -o $fileNameWithoutExt && $dir$fileNameWithoutExt"
```
2) as a list
```json
"powershell": [
"powershell",
"-ExecutionPolicy",
"ByPass",
"-File"
],
```
3) as a list of list, for complex commands:
```json
"cpp-memory": [
[
"cd",
"$dir"
],
[
"clang++",
"-g",
"-O1",
"-std=c++17",
"-Xclang",
"-flto-visibility-public-std",
"-Wall",
"-Wextra",
"-Wshadow",
"$fileName",
"-o",
"$fileNameWithoutExt.exe"
],
[
"$dir$fileNameWithoutExt.exe"
]
]
```

(this is slightly related to #291 )