Fixed a syntax highlighting issue of markdown

Fixed a markdown syntax highlighting issue. Now, **strong** and __strong__ show bold and *em* and _em_ show italic correctly. 

Text objects ah and ih now support markdown header starting with #. And vih now selects only contents, leaving the header and blank lines unselected

Added some header motion. Note: Only atx style header of markdown starting with # and header of default syntax starting with = are supported
]u and [u - go to upper level header
]c and [c - go to current header
]] - go to next header
[[ - go to previous header
]= - go to next sibling header
[= - go to previous sibling header