Implement activate function

For clever motion & highlight
Implement g:Easymotion_add_search_history option
Implement 'n' key motion offscreen search!

Including some refactoring
    - s:Easymotion function
        - handling original & cursor position
    - autoload/EasyMotion/command_line.vim
    - s:Getvisualstartposition function
Update doc: enhancement of 'n' key motion
Test: add test case for off-screen search
Implement scroll to find motion cmd line
Fix typo
Fix test
Merge branch 'feature/offscreen_search' into master