Delegate convert_with_options to convert method

There’s the change that we’ve discussed in [this](https://github.com/asciidoctor/asciidoctor/commit/d98c8e9cd5e5793fdab6da7a0a6f37eb8349878b#commitcomment-8359933) thread. I’ve run userguide-loop and mdbasics-loop benchmarks on MRI 2.1.2 and there’s no statistically significant difference in performance.

### Before

**userguide-loop (50x):**
Run Total: 19.358426
Best Time: 0.343773

**mdbasics-loop (50x):**
Run Total: 19.917844
Best Time: 0.346665

### After

**userguide-loop (50x):**
Run Total: 19.375733
Best Time: 0.339852

**mdbasics-loop (50x):**
Run Total: 19.79033
Best Time: 0.349473

Should I run some other benchmarks on it?