Fix optional parameters parsing for \newcommand

[PHPirates]

Summary of additions and changes

• Fix optional parameters parsing when first parameter for new command is optional

How to test this pull request

    \newcommand{\test}[2][1]{hi}
    \newcommand{\lvec}[2][n]{\ensuremath{#2_1+\cdots + #2_{#1}}}
    For the series \lvec{x} we have \[ \lvec{x} = \sum_{k=1}^{n} G_{\lvec[k]{y}} \]
    \test{asfd}{asdf}
    \lvec{}{} % still wrong: first param should be optional. may be fixed in a different branch