Support command line options in lieu of property notation

[spboyer]

Use --option in addition to or instead of property notation /p:

/p:CollectCoverage=true -> --collect|-c (true|false)
/p:CoverletOutputFormat=opencover -> --output|-o (json (default), lcov and opencover)
/p: CoverletOutputDirectory --> --directory|-d

[tonerdo]

This is unfortunately a limitation with MSBuild. The standard command line option format will be supported by the coverlet cli once it's release (should coincide with the general availability of .NET Core global tools)