feat: tags

[Pinjasaur]

closes #10

This adds tag support as a means of organizing entries (effectively just posts).

I landed on a tags: ... string immediately below the title:

# My Title
tags: one, and-two, and-even-three

And the rest of the normal body goes here.

Internally, the mappings get stored as associative arrays. The posts get delimited by | characters, so those are now invalid characters in a filename and will cause bic to error out at build-time.

All the tags are available on a tags.html view and also use a tag.html and __tag.html to create tags/${tag}.html views to showing the mapping of individual tags to posts.

Notes:

• Drafts and pages cannot be tagged (drafts makes sense, pages could be an enhancement in the future)
• Not extensively tested nor documented

[Pinjasaur]

cc @mfossen

[Pinjasaur]

Need to also update the sitemap.xml.