Render tag cloud on tags page

[sio]

Tags page was quite boring and didn't use horizontal space efficiently. This commit replaces plain tag list with tag cloud that gracefully scales to any screen (Attila's responsive design principle is not violated).

Details:

• Tags are still sorted alphabetically
• Tags with more articles have bigger font size
• Number of font size steps is defined via TAG_CLOUD_STEPS variable. If that variable is not set or is zero, default value of 5 steps is used. Stylesheet is written to support up to 10 steps.
• Tag tooltip shows number of articles with that tag

There exists a separate plugin for tag cloud, but using it still requires making changes to the theme. Trivial math calculations can be done as easily in Jinja as in Python, so the dependency on external package can and should be avoided.

Screenshots: before, after

[arulrajnet]

@sio please update this setting about in README. Otherwise LGTM

[sio]

I have added the relevant section to README.