Feature export as markdown

[brandonchupp]

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] Tests for the changes have been added (for bug fixes / features)
  • Due to the nature of these changes, I believe they can't easily be tested by unit tests. The other export formats are do not have tests either (since they would be integration tests). If you have a suggestion for what to test, I'd be happy to add it.
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Other... Please describe:

What is the current behavior?

At the moment, the repository supports exporting the code review as HTML, JSON, and a few other formats.

What is the new behavior?

These changes introduce a new export format for Markdown. Additionally, they allow the user to specify a custom markdown file template to use for the export.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

Additionally, these changes include two bug fixes:

• Show error message when provided default template is not found
• Fix broken link on README.md

[d-koppenhagen]

@all-contributors add @brandonchupp for code

[allcontributors[bot]]

@d-koppenhagen

I've put up a pull request to add @brandonchupp! :tada: