add koreader plugin mechanism
This should implement #505.
add Evernote plugin to export highlights and notes

The "My Clipping" file that storing highlights and notes for Kindle
native readers could also be parsed and exported. The parser is
implemented in `evernote.koplugin/clip.lua`.

Parsed highlights and notes in one book will be packed and rendered
into html node with a slt2 template `note.tpl` that complies with
evernote markup language(ENML).

Finally the evernote client will create or update note entries and
push them to Evernote cloud.
update pot file for plugins
Merge pull request #556 from chrox/master

add Evernote plugin to export highlights and notes