Support .net core portable pdb format

[sawilde]

Please provide the following information when submitting an pull request, where appropriate replace the [ ] with a [X]

The issue or feature being addressed

Support .net core portable pdb format #595

Details on the issue fix or feature implementation

Use mono.cecil/master to handle new pdb format

still awaiting official 0.10.* release from mono.cecil before merging to mainline

Confirm the following

• I have ensured that I have merged the latest changes from the main branch (or which ever branch is appropriate) from opencover/opencover
• I have run build create-release locally and have encountered no issues
• I agree to follow up on any work required to resolve any issues identified whilst my request is being accepted

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This change is 

[adityapatwardhan]

@sawilde I am waiting on support for portable pdb format for use in our project https://github.com/powershell/powershell. I have tried the change from https://ci.appveyor.com/project/sawilde/opencover/build/4.6.589/artifacts and it works great. I believe you are waiting on mono.cecil to release a 0.10.* version to release this change. They have a beta release available at: https://github.com/jbevain/cecil/releases/tag/0.10-beta1

Will be possible to have a beta release for OpenCover as well, taking dependency on mono.cecil beta?