[FEATURE REQ] provide a time period to run `export` command for

[MadhavJivrajani]

Is your feature request related to a problem? Please describe.
Along with providing an option for a number of iterations to run for at a specific refresh rate, it would be helpful to provide a total run time and a refresh rate at which this should happen.

Describe the solution you'd like
grofer export -t 120 -r 500
Total time period -t can be added as a non-persistent flag and units would be in seconds.

If the -t flag is used then the parameter for iteration should be explicitly calculated using total time and refresh rate.

This is done with the assumption that data fetch is almost instantaneous in nature which is a non-realistic yet reasonable assumption to make imo.

To be worked on after #34

Development for export feature is being done on the export branch (link)