Add bold, italics, bold-itlatics, and monospaced font

[ianyepan]

The CMU font package actually comes with all these fonts. We'd love to have them included so Typora doesn't use its default bold/italics rendering.

[ianyepan]

@font-face {
  font-family: "Computer Modern";
  font-style: normal;
  font-weight: normal;
  src: url('./paper/cmunrm.ttf');
}

@font-face {
  font-family: "Computer Modern";
  font-style: italic;
  font-weight: normal;
  src: url('./paper/cmunti.ttf');
}

@font-face {
  font-family: "Computer Modern";
  font-style: normal;
  font-weight: bold;
  src: url('./paper/cmunbx.ttf');
}

@font-face {
  font-family: "Computer Modern";
  font-style: italic;
  font-weight: bold;
  src: url('./paper/cmunbi.ttf');
}

I managed to put the ttf files in my local machine and tweaked the css. It worked -- please consider pushing a commit to enhance the feature.