Add support for markdown as an alternative to HTML for ApexDoc and overview content

[SCWells72]

ApexDoc comments and the user-specified overview content (-f/--overview argument) are based on HTML. Given the prevalence of markdown, it would be great if that could be used for all content authoring instead of/in addition to HTML.

[SCWells72]

Markdown-to-HTML libs:

• https://github.com/commonmark/commonmark-java
• https://github.com/vsch/flexmark-java
• https://github.com/JetBrains/markdown

[SCWells72]

After tinkering with this for a bit, I'm starting to think that perhaps full markdown support for documentation comment text isn't necessarily as important as support for some of the most common markdown text styling and simple formatting syntax, specifically the following:

• Code via `text` - already supported
• Bold text via **text**
• Italic text via _text_ or *text*
• Strikethrough text via ~~text~~
• Heading text via # text, ## text, and ### text
• Block quote via > text
• Link via [Text](url) - including references to type/member declarations
• Fenced code block via ```text``` - need to determine if there's a simple way to add language-specific syntax highlighting
• Ordered list via 1. text - likely single-level for simplicity until I hear otherwise
• Unordered list via - text or * text - likely single-level for simplicity until I hear otherwise

I would then also support markdown-based overview files and could convert them to HTML using either the simple markdown syntax support described above or one of the aforementioned Markdown-to-HTML libs that provide more complete support. I think starting with the more simple version should be sufficient until/unless I hear otherwise.

[SCWells72]

Delivered in 1.0.0.5.