Disable Swift 5.3 workaround for 5.4 and higher (JohnSundell#67)

Add support for Markdown Tables (JohnSundell#44)

This change makes Ink support Markdown Tables

Co-authored-by: Christian Mitteldorf <cmit@danskebank.dk>
Co-authored-by: John Mueller <jmuellerokc@gmail.com>
Co-authored-by: John Sundell <john@sundell.co>

Fix allow whitespaces check in Reader.read(until...) (JohnSundell#41)

Enable metadata keys and values to be modified (JohnSundell#39)

This change introduces two new `Modifier.Target` instances: `metadataKeys`
and `metadataValues`, which can be used to customize how Ink parses metadata.
This in turn enables Publish plugins to be written that bridges the gap
between Publish’s built-in metadata format and that of external tools, such
as other static site generators or CMSs.

Markdown: Add title property (JohnSundell#28)

This change adds a `title` property to the `Markdown` struct, which when
accessed is lazily evaluated to compute a plain text version of the first
top-level heading (H1) found in the Markdown text.

To make this happen, a `PlainTextConvertible` protocol is introduced,
which is also made a requirement of `Fragment`. A best effort is made
to convert each fragment into a reasonable plain text representation,
even if many of them will never be used as the implementation currently
stands (for example, lists can’t be placed inside headings). It still
felt worth it to do a proper implementation in case we ever make this
a public API for some reason.

This lets tools built on top of Ink extract a title for the parsed Markdown
document without doing additional string parsing.

Introduce case-insensitivity for reference link labels (JohnSundell#16)

Merge PR from @ezfe
* Introduce case-insensitivity for reference link labels
* Moved `.lowercased()`

HTML: Treat comments as self-closing tags (JohnSundell#10)

This patch makes Ink parse HTML comments correctly, by treating them as
self-closing tags, rather than as opening ones (which would cause all
subsequent text to be parsed as plain HTML).

Merge pull request JohnSundell#1 from JohnSundell/linux-ci

Linux: Include all test cases

Add LICENSE file