allow multiple values in query strings

[AnnMarieW]

This pull request allows for multiple values in query strings to be passed to the layout function.

For example, this URL has 2 values for indi:

http://127.0.0.1:8050/chart?com=AAA&indi=sma-50&indi=sma-200

The following is now passed to the layout function. Note that the value for indi is a list

{'com': 'AAA', 'indi': ['sma-50', 'sma-200']}

Previously, only the first value was included:

{'com': 'AAA', 'indi': 'sma-50'}

[AnnMarieW]

Yippee - Thanks!