Parsing super/subscript is different from LaTeX

[amuramatsu]

Some expressions with ^ and/or _ are rendered different from LaTeX.

X_12 expected: <mrow><msub><mi>X</mi><mn>1</mn></msub><mn>2</mn></mrow> actual: <mrow><msub><mi>X</mi><mn>12</mn></msub></mrow>

X_\mathrm{min} excepted: <mrow><msub><mi>X</mi><mrow><mo>m</mo><mo>i</mo><mo>n</mo></mrow></msub></mrow> actual: <mrow><msub><mi>X</mi></msub><mrow><mi>m</mi><mi>i</mi><mi>n</mi></mrow></mrow>

[roniemartinez]

@all-contributors please add @amuramatsu for bug

[allcontributors[bot]]

@roniemartinez

I've put up a pull request to add @amuramatsu! :tada: