Add support for attributes on links, images, and inline code spans

[Witiko]

Closes #123.

Tasks

• Complete link attribute support from jgm#36, see jgm#43. Add link attributes syntax extension (see also Pandoc).
• Add inline code attributes syntax extension (see also Pandoc).
• Add link and inline code attribute context renderers (see also Pandoc).
• Add linkAttributes and inlineCodeAttributes Lua options.
  See example for linkAttributes Lua option in Image resizing  #50 (comment).
  Load the corresponding syntax extensions in M.new().
• Add unit tests for inline code and link attributes.
• Add default LaTeX renderers for image attribute contexts.