Add Markdown Support

[hlavu]

Hi Quan, I have completed the code for the markdown support.
Here are some changes I had made:

• In mini-ssg.js:

  • Modify the file handling so it will accept the ".md" extension.

• In generateHTML.js:

  • Render Heading 1.
  • Render Heading 2.
  • Render Link.

• In README.md:

  • Update README.md to add documentation for Markdown support.

Please review it and let me know If you want me to change any part.

[mqnguyen5]

Overall, the pull request looks great. However, there are some things that should be changed and I have left comments so you can have a look.

[hlavu]

That's great, I will change them.

[mqnguyen5]

Everything looks great! Thanks for the contribution.