Add support for universal (cross-format) themes

[Witiko]

Closes #276.

Tasks

• Move options import/theme from LaTeX (Section 2.3.2.3 of the technical documentation) to plain TeX (Section 2.2.2.5).
  • The documentation for options import/theme was moved in commit 3261f9f from this PR.
  • The options import/theme were moved to the plain TeX interface in commit 8253bb4 from this PR.
• Define function \@@_load_theme:nn not only in LaTeX (Section 3.3.2.1 of the technical documentation), but also in plain TeX (new Section 3.2.2).
  • Prevent repeated loading of plain TeX themes.
• Move theme witiko/tilde from LaTeX (Section 2.3.2.3 of the technical documentation) to plain TeX (new Section 2.2.2.5).
• Move options snippet/import from LaTeX (Section 2.3.2.4 of the technical documentation) to plain TeX (new Section 2.2.2.6).
• Move option code from LaTeX (Section 3.3.3 of the technical documentation) to plain TeX (Section 2.2.6).
• House-keeping of the technical documentation:
  • Move sections 2.2.2.5 and 2.2.2.6 into higher-level sections 2.2.3 and 2.2.4.
  • Move Section 2.3.2.3 of the technical documentation into a new higher-level Section 2.3.3.
  • Move Section 3.3.2.1 of the technical documentation into a new higher-level Section 3.3.4.
• Add macro \markdownSuper, which loads the plain TeX theme from the LaTeX theme.
  The macro \markdownSuper was documented in commit 3261f9f from this PR.
• Rename \markdownSuper, which is a super confusing name, to \markdownLoadPlainTeXTheme.
• Log whether a plain TeX or a LaTeX theme is being loaded.
• Add support for ConTeXt themes by analogy with LaTeX themes at this point of the implementation.
• Move default LaTeX, ConTeXt renderer (prototype) definitions to a new theme witiko/markdown/defaults.
• Merge files plain-setup.tex and the LaTeX theme witiko/markdown/test into plain TeX theme with the same name.
  Create stub ConTeXt and LaTeX themes that use \markdownLoadPlainTeXTheme to load the plain TeX theme.

Here are some further remarks:

• After the beginning of a document, loading a theme in LaTeX should emit an error if an *.sty file exists and load a *.tex file if it exists or emit an error otherwise.