Add ability to process units of px in \\[...] (not particularly accurate, however).  Issue #890
Make px-per-inch configurable and use 72dpi (pt = px in this case)
Fix spacing of open delimiter from \left...\right.  Issue #889.
Allow \newenvironment to provide an optional argument.  Issue #881
Allow commas in place of decimals in TeX dimensions.  Resolves issue #856.
Combine multiple elements into a single typesetting pass.  Issue #873.
Merge branch 'issue890' into develop.  Issue #890.
Merge branch 'issue889' into develop.  Issue #889.
Merge branch 'issue881' into develop.  Issue #881.
Merge branch 'issue856' into develop.  Issue #856.
Merge branch 'issue873' into develop.  Issue #873.