Add widget MetricsEvolCustomized to display more than a repo per chart

src/Convert.js:
 Add a new function called convertMetricsEvolCustomized to be added
 to the callback when the second set of JSON files has been
 downloaded.

src/Viz.js:
 Add functions to (displayMetricsLinesRepos) process the data before
 plotted in order to get data for different repos and the same metric.
 Changes also add more modular logic to function displayDSLinesZoom