[TOOLS] Create regex interface for getting files from remote FS in mongofiles

Description

Specify new subcommand `mongofiles ... get_regex ...` in documentation

Engineering Description
Query files in GridFS by regex to match filename. Command is of the following form:

`mongofiles ... get_regex "<regex>" --options "<regex-options>"`

Here, `<regex-options>` should specify options available to the $regex query.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)