Fix bug and a little new feature

## Fix automatic header id bug
Add multi bytes chars support in header ID

## Fix alternate files sort bug
The sort work incorrect when alternates more than 1, remove the sort, and use `index` to achieve this

##Add follow links keyboard module
get the link at cursor, and check the link address:

* web url: open it
* header id: find the specified header, support automatic header ids
* local file: check if there has alternate files, and check weather it could be open in vim, if the file does not exist, can also make it(a little like vimviki)