manual linebreaking and indenting/supporting mo attrs

The is related to w3c/mathml-core#127, but I feel that w3c/mathml-core#127 should (and so far is) about automatic linebreaking. However MathML 3 also has a number of attributes on mo that can be used for forced linebreaks and indentation. These can be useful for breaking at an = and aligning subsequent lines on them as in:

The linebreaking attributes from MathML 3 are given here; the indentation attributes are in the next subsection. To support the above functionality, the attributes that are needed are:
linebreak= 'newline'
linebreakstyle="before" | "after" | "duplicate" | "infixlinebreakstyle" (only before/after are high priority)
indentalign = "id" (other values are "left" | "center" | "right" | "auto" but not needed for manual indents)
indenttarget = idref

Maybe supporting indentshift would be useful also and easy to implement (just add to the left padding).

Supporting these attributes do not require any additional passes. Adding this support would be a good way to begin to support linebreaking.

Start with the MathML 3 linebreaking and indentation attribute sections linked in the issue, and compare the requested behavior with w3c/mathml-core#127. Done means supporting linebreak="newline", before/after linebreakstyle, indentalign="id", and indenttarget, with indentshift considered if practical.