Markdown support

[TueeNguyen]

This patch fixes #4

It supports correctly formatted markdown files with bold, italic, heading and link.

Refactor code by adding a function writeHtmlFile(title, body, file, fileType

[menghif]

Thank you Tue, This looks great!
One thing I noticed is that it only works with Markdown files when a directory is selected as input. It does not work when a Markdown file is selected as input.

You just need to add a line similar to the one that looks for .txt files:

if (fs.statSync(argv.input).isFile() && argv.input.match(/.txt$/)) {
    files[0] = argv.input;
  }

You can also update the README to mention that the program now works with Markdown files and you can remove textfiles/text.md from the commit (I will add an example myself to the examples branch)