Issue 336: Replace new line with <br> in markdown tables

[wimspaargaren]

Addressing the issue of incorrect new lines in markdown documentation by replacing new lines with<br> tags.

Original issue: https://github.com/pseudomuto/protoc-gen-doc/issues/366

Related PRs addressing the same issue:

This one is actually pretty recent, but escapes the break tags:

• https://github.com/pseudomuto/protoc-gen-doc/pull/486

This one seems to have a failing build:

• https://github.com/pseudomuto/protoc-gen-doc/pull/445

This one's missing the examples:

• https://github.com/pseudomuto/protoc-gen-doc/pull/386

[wimspaargaren]

@pseudomuto Please let me know if this PR needs any additional changes. Would be nice if we can fix this one 💪 !