Add markdown summary report

[efenderbosch-atg]

What is your use-case and why do you need this feature? I'd like a kover/report.md that would be appropriate for use as a PR comment via github actions, just the "Overall Coverage Summary" from the HTML report would be great.

Maybe I can grab the HTML? Git hub will format it like this:

Package	Class, %	Method, %	Branch, %	Line, %	Instruction, %
all classes	97.5% (79/81)	94.3% (181/192)	89.4% (93/104)	98.4% (569/578)	98.4% (3388/3443)

[efenderbosch-atg]

As markdown it could look like

foo-project: Overall Coverage Summary

Package	Class, %	Method, %	Branch, %	Line, %	Instruction, %
all classes	97.6% (80/82)	94.1% (176/187)	88.8% (87/98)	98.5% (543/551)	97.2% (3258/3351)

[efenderbosch-atg]

For now, I'm converting to markdown w/ pandoc and grabbing the summary w/ sed as a workaround:

pandoc build/reports/kover/html/index.html -f html -t gfm | \
  sed -n '/# foo-bar-service: Overall Coverage Summary/,/## Coverage Breakdown/p' | \
  grep -v '## Coverage Breakdown' >> $GITHUB_STEP_SUMMARY

[shanshin]

Closed in favor of https://github.com/Kotlin/kotlinx-kover/issues/605