fix bug about dir and main page

redirect to main page if editor is requested on dir
fix findSyaroDir()

search working dir and don't search GOPATH
fix Table layout in Markdown
fix editor

markdown css
preview area padding
save modal
change markdown parser to marked.js

no longer use blackfriday
change title searching method in page
change some func's name
convert markdown to html when page loaded
processWikiLink() use string instead of byte array
add MathJax and highlight.js

add --mathjax, --highlight command line options
enable MathJax in Page and Editor when --mathjax option is enabled
enable syntax highlight in Page and Editor when --highlight option is enabled
realtime math rendering and code highlighting in Editor
add option dropdown for Editor
update README