Fix macro for Zeppelin

[jeremyrsmith]

• Fixes macro for Zeppelin (wrong type signature of DisplayUtils.html)
• Update docs to describe how ShowRender is used in manual cases

[codecov-io]

Codecov Report

Merging #99 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #99   +/-   ##
======================================
  Coverage    70.3%   70.3%           
======================================
  Files          27      27           
  Lines        1785    1785           
  Branches        6       6           
======================================
  Hits         1255    1255           
  Misses        530     530

Impacted Files	Coverage Δ
...src/main/scala/vegas/macros/ShowRenderMacros.scala	0% <ø> (ø)	:white_check_mark:

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[dbtsai]

LGTM. Thanks.