Clickable step indicators + HTML code rework

[earshinov]

PR #143 rebased onto develop branch.

[madoar]

@earshinov the layout changes to the vertical layout lead to some visual errors:

• the step titles are not vertically centered
• the circle of the step indicators are distorted on the y-axis (vertically)

An example:

[earshinov]

@madoar , Ouch! I will try to get to it next week.

[madoar]

Thanks :)
After this bug has been fixed I'm planning to publish a new release (i.e. version 4.0.0) to npm.

[earshinov]

@madoar , I created new #159. Please have a look.