Fix not allowing from token as market/country

[Qluxzz]

Description

Fixes "from_token" not being a valid option for market/country.

Create Market enum which can be a country or from_token in order to satisfy both conditions.

Update usages of Country to instead use Market enum.

Update mentions and parameters of country/market to be consistent with spotify API docs.

Fixes #28

Motivation and Context

To allow market to be automatically selected from the users access-token instead of having to be supplied manually.

Dependencies

Type of change

Please delete options that are not relevant.

• This change requires a documentation update
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Please also list any relevant details for your test configuration:
I ran all tests but the most interesting ones would be the following since they used the old Country enum

• Market::Country: test_artist_top_tracks
• Market::Country: test_categories
• Market::Country: test_category_playlists
• Market::Country: test_recommendations
• Market::Country: test_search_artist

I've also added a test that uses Market::FromToken functionality, but since the market supplied is not static and changes with the oauth token credentials, it maybe wouldn't be such a good test?

• Market::FromToken: test_search_artist_with_from_token

[marioortizmanero]

I think the test you added is OK, since we can't check the results anyway. As long as it makes sure it doesn't break it's enough for now.

[ramsayleung]

Is this PR is ready to merge, it seems there is no more update after the last commit?

[Qluxzz]

Yes, I implemented the to_string trait instead of into trait as @marioortizmanero asked me about, and then requested a review.

[marioortizmanero]

Yes sorry! I was busy but I'm finally done with finals. Merging this, thanks for the contribution :)

[Qluxzz]

@marioortizmanero no worries, hope you aced your finals!