Added JSON and CSV file providers

[savagedev]

This implements #55 to support local file providers with a JSON or CSV file format.

• Larry now has a new contentfile provider which supports files in the CSV and JSON formats
• This provider requires a command line arg --content-file which provides the local file and the file extension indicates the format of the file
• The provider expects the Content struct fields on each record where only the Title field is required
• The Provider uses the reservoir sampling method to efficiently (one pass) find a random record in the file
• The provider also has an optional command line arg --skip-csv-header to skip past the header record of the CSV file
• The CSV file can have an "unlimited" number of ExtraData fields by just including them at the end of each record (note: every CSV record needs the same number of fields.
• The CSV file can have fields surrounded by double quotes or not
• The readme has been updated to include the new features

Update: I updated this comment to reflect the refactoring changes I made

[savagedev]

@ezeoleaf While working on the S3 file source I see where this code can be refactored quite a bit to eliminate redundant code. You can either close this PR and I'll create a new one or just wait and I'll update this one with my changes.

[savagedev]

@ezeoleaf My refactoring is complete!

[ezeoleaf]

Nice work!