Added a new "Themes" tab to choose color theme for charts

[sgologuzov]

I've added a new tab "Themes" to allow user to change color theme of the chart. This tab contains a few standard highcharts themes (https://github.com/highcharts/highcharts/tree/master/js/themes) plus a greyscale theme required for reports printing.

Here is an example image: https://www.dropbox.com/s/m4ie3081lwibryx/Screenshot%20from%202016-11-01%2000%3A08%3A52.png?dl=0

[daemth]

Hi Sergey, Thanks again for another pull request! This all looks good to me, I only got two small proposals for improvement:

• for now I'd like to keep the themes-tab as optional (just like the chartTab: https://github.com/sgologuzov/easychart/blob/master/src/js/app.js#L127) in order not to break existing (Drupal-)implementations
• I guess it would be useful to provide a default theme too, that would make it easier for users to "undo a chosen theme"

[sgologuzov]

Hi Thomas, Thank you for your review. I've fixed your notes. Please review it again.