Namespace fixes

[DylanPiercey]

Description

This PR moves the namespaceURI calculation from compile time to runtime which fixes a number of edge cases around the dynamic tag, ambiguous tags, or explicitly setting the xmlns attribute.

Fixes #1098

Also adds support for MathML.

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[coveralls]

Coverage decreased (-0.01%) to 90.524% when pulling 69dedc6 on namespace-fixes into b82a867 on master.

[codecov]

Codecov Report

Merging #1333 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1333      +/-   ##
==========================================
+ Coverage   90.53%   90.62%   +0.09%     
==========================================
  Files         340      340              
  Lines       12445    12632     +187     
==========================================
+ Hits        11267    11448     +181     
- Misses       1178     1184       +6

Impacted Files	Coverage Δ
src/compiler/ast/HtmlElement/index.js	92.77% <ø> (-0.49%)	⬇️
src/core-tags/components/preserve-tag-browser.js	100% <ø> (ø)	⬆️
...c/compiler/ast/HtmlElement/vdom/HtmlElementVDOM.js	98.14% <100%> (+0.94%)	⬆️
src/runtime/vdom/morphdom/specialElHandlers.js	80.55% <100%> (ø)	⬆️
src/compiler/CompileContext.js	93.54% <100%> (+1.25%)	⬆️
src/runtime/vdom/morphdom/index.js	98.16% <100%> (+0.11%)	⬆️
src/runtime/vdom/AsyncVDOMBuilder.js	92.03% <100%> (-0.12%)	⬇️
src/runtime/vdom/VNode.js	100% <100%> (ø)	⬆️
src/runtime/vdom/VElement.js	97.53% <100%> (+0.29%)	⬆️
src/compiler/config.js	71.42% <0%> (-3.58%)	⬇️
... and 4 more

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