Several improvements on docs target

[dahlia]

• 7ac8e65: Added the missing module-level docs.
• f0c3e30: Show which tag (if exists) of a union is default. The default keyword on tags is a recently added feature; see also Optional default tag for union type #227 and Optional default tag for union types #13.
• 85aedff: Adjusted on HTML markups; it's about styles and semantics.
• dc13fd9: Require the latest tox version to avoid recent breakage on Python 2.7 builds. (I guess it's related to the migration from https://pypi.python.org/ to https://pypi.org/ but am not sure.)

[checkmate-bot]

Checklist 🤔

tox.ini

• When the list of supported Python versions change, synchronize the change with .travis.yml and appveyor.yml too.

[codecov]

Codecov Report

Merging #259 into master will increase coverage by 0.09%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   75.75%   75.84%   +0.09%     
==========================================
  Files          33       33              
  Lines        2409     2414       +5     
  Branches      132      132              
==========================================
+ Hits         1825     1831       +6     
+ Misses        452      451       -1     
  Partials      132      132

Impacted Files	Coverage Δ
src/Nirum/Docs.hs	58.9% <100%> (+2.38%)	⬆️
src/Nirum/Targets/Docs.hs	65.3% <75%> (+0.35%)	⬆️
src/Nirum/Constructs/Module.hs	65.9% <0%> (+2.27%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0b08904...dc13fd9. Read the comment docs.