Define a variable to change the size of text in guide buffer.
Define a function to calculate the height or widht of popup window.
Use `guide-key/popup-window-size' to calculate the size of popup window.
No more need `with-current-buffer'.
Correct indent
Add documentation about `guide-key/text-scale-amount'.
Synchronize README and the commentary of code.
Calculate the number of items per line in `guide-key/insert-following-key'.
Change the margin of popup window according to that position.
Implement a feature to scale text size.

Close #4
Bump a version to 1.2.0.