CLI flags for easier crawler handling

[rajivharlalka]

Is your feature request related to a problem? Please describe.
Add cli command flags to handle cases such as downloading papers for specific year, limiting the number of parallel colly connections.

[proffapt]

@rajivharlalka or @harshkhandeparkar please update the state of this issue to be reflected on the kanban.

[harshkhandeparkar]

@dipamsen Would this issue be handled in the crawler overhaul or a followup? If yes, I'll assign this to you.

[dipamsen]

@harshkhandeparkar sure