Make our implementation of attributes compatible with jgm/pandoc

[Witiko]

Our implementation of attributes differs from the implementation in Pandoc.
Here is a list of the differences in a decreasing order of importance:

• Besides alphanumeric characters, Pandoc also allows -_:. to appear in an identifier, whereas we only allow -_.
  This prohibits users from assigning colon-delimited identifiers to elements, e.g. # Introduction {#sec:introduction}.
• For key-value pairs, Pandoc allows values to be enclosed in ASCII quotation marks or apostrophes, wheras we don't.
  This prohibits users from typing values that contain spaces.
• Key-value pairs in the form id=VALUE are treated as though #VALUE were specified.
• Key-value pairs in the form class="VALUE1 VALUE2 VALUE3" are treated as though .VALUE1 .VALUE2 .VALUE3 ... were specified.

Continues #164 and #177.

[Witiko]

Also update the link-attributes.test testfile according to Pandoc's manual, so that we use a key-value pair in the form class="VALUE1 VALUE2 VALUE3".