Update ct_search() to allow for "All of yyyy" queries

[ChrisMuir]

This was brought up by GH user pedromein in issue #12. The Comtrade API has a (new?) feature that allows for queries that return monthly data (vs annual data) for an entire individual year. Previously, if searching for monthly data, there were two options:

• Pass all to start_date and end_date to get all monthly data for all years.
• Pass a date range, in months, to start_date and end_date. Doing this, the most you could get from a single query was five consecutive months worth of data.

Now they have an option to get all monthly data for a single year. So need to update ct_search() to give users this option.

[ChrisMuir]

Function ct_search() is now able to utilize the All of yyyy feature of the Comtrade API (added via commit 8f6f3bf). The function can now pull all monthly data for a given year in a single query.