Add initial support for Markdown

[tcvan0707]

Fixes Issue #2:
In pgprogram.cpp :

• Modify if statement inside main to check .md files.

In HTMLFile.cpp :

• Add if statement inside the for loop to create Markdown syntax feature for Heading 1
• Using find to find the index of the mark #
• Then using erase to delete the mark # at a certain position.
• After all, append the <h1> tag to the string just removed #

In README.md :

• Add additional documentation for Markdown support.

[kiennguyenchi]

Hi Thanh, I have suggested few edits before pulling the request.
1/ In pgprogram.cpp, in main function, rewrite the if-else loop to check for folder or single file. With this code, it will make the program create a folder with a single .md file, which is not good.
2/ In HTMLPage.cpp, in setHTMLBody function, we work with vector 'textFile', not variable 'file'. You should create an if-else in the for-loop. If a line is heading identifying by #, add paragraph. Else not, add paragraph to each line.
Thank you!