Allow popup function to be called directly

This will allow me to integrate guide-key into ergoemacs-mode.

https://github.com/ergoemacs/ergoemacs-mode/issues/15
Merge pull request #11 from mlf176f2/patch-1

Allow popup function to be called directly
Allow key sequences with regexp special characters
Merge pull request #12 from mrc/master

Allow key sequences with regexp special characters
Remove an argument for regexp for highlight
Define a function to get a face for highlighting command
Use `guide-key/get-highlight-face' instead of `guide-key/propertize-string-according-to-command'
Delete `guide-key/propertize-string-according-to-command'
Change the type and the default value of `guide-key/highlight-command-regexp'
Modify a procedure to add regexp along with its type
Modify the docstring of `guide-key/highlight-command-regexp'
Modify a link of one-key to more explanatory page

Close #13
Modify a link of one-key to more explanatory page
Bump a version to 1.2.3
Modify a docstring
Merge branch 'multiple-highlight'

Implement the feature to highlight multiple keywords (ref. #14).
Close #14.