[FEATURE] Alt image text if set

Alternative image text value if set. If not then image name is used.
[BUGFIX] Option header.types adjusted to W3C spec

According to W3C, only 1-6 are valid levels

By this http://www.w3.org/wiki/HTML/Elements/h1,_h2,_h3,_h4,_h5,_and_h6
there should be only header types 1-6.