Enable jit-analyze to take multiple metrics as input and add an option to output .md file

[kunalspathak]

I was tired of gathering the diff data for individual metric by invoking jit-analyze multiple times, so I went ahead and added ability to specify multiple metrics in comma-separated form so we can see the diffs of multiple metrics at one instead of invoking jit-analyze multiple times. Another motivation was that I wanted to output the diff result in .md file so I can just copy the contents from it and paste it on GH. So I went ahead and added an option of --md that will output the diff result (of all the metrics specified) in an .md file.

Example of what you end up getting in .md can be seen in https://github.com/dotnet/runtime/pull/47307#issuecomment-765046091.

Contributes to https://github.com/dotnet/jitutils/issues/125

[kunalspathak]

@dotnet/jit-contrib , @AndyAyersMS