POSIX compliance

According to the POSIX standard[0], the length() function only supports strings.
Although most common awk implementations support array length() as an
extension[1], it is better to make the program portable.

GNU Awk has a --posix flag that ensures that the script is POSIX compliant.
Running markdown.awk with this flag results in an error. This can be fixed by
getting the length of the array returned by split().

[0]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html
[1]: https://www.gnu.org/software/gawk/manual/html_node/Common-Extensions.html

Note the support of tables in readme

Add support for tables

Add LICENSE file

Automatically skip metadata headers

Add a shebang to run markdown.awk as script

Add support for BSD awk

Add support for better bold/italic formatting

Better support for horizontal rules

Add support for images

Add support for horizontal rules

Lists within blockquotes and tabs in lists

Add more edge cases to code and header handling

Implement links

Fix off-by-one error in inline code block parsing

Add proper support for escape sequences

Better support for bullet lists

Add feature list to README.md

Add simple handling of html tags

Properly escape "<", ">" and "&"