Adds an option for specifying an output file when using markdown-it from the command line.
I'm on the VScode team and in our docs we currently recomend that users install marked for converting markdown to html: https://code.visualstudio.com/Docs/languages/markdown#_compiling-markdown-into-html . I'd like to use markdown-it in this example instead, but correctly setting up a task for this is not as easy as it is with marked. Having a commandline output file option addresses this.
Calling `process.exit()` can cause process to abort before the entire text
is written to the stdout.
When I tested this, out of 1.8Mb binary only 65536 bytes had been written.
Vitaly Puzrin