Move plotly to Imports

[aravindhebbali]

Current Behavior

User installs the rfm package, plotly is not installed as it is not included in Imports. When any of the below functions are called with the interactive argument set to TRUE:

• rfm_plot_segment()
• rfm_plot_segment_summary()
• rfm_plot_segment_scatter()
• rfm_plot_revenue_dist()
• rfm_plot_median_recency()
• rfm_plot_median_frequency()
• rfm_plot_median_monetary()
• rfm_plot_heatmap()
• rfm_plot_histogram()
• rfm_plot_order_dist()

the package checks if plotly is already installed. If yes, it generates the plot, else, it prompts the user to install plotly.

New Behavior

Move plotly to Imports. This ensures that the user need not be bothered to install plotly if its not already installed.