Set the baseurl in the constructor, and make it filterable

[boblinthorst]

Describe the problem (reference the issue if applicable) and what the PR accomplishes.

#184

In this PR, I make the baseUrl filterable, so that the baseurl can be changed to https://api.sandbox.taxjar.com.

Steps to Reproduce

1. Do this
2. And that
3. Finally this

Expected Result

After applying the PR, this should happen.

Click-Test Versions

• Woo 4.0
• Woo 3.9
• Woo 3.8
• Woo 3.7
• Woo 3.6
• Woo 3.5
• Woo 3.4
• Woo 3.3
• Woo 3.2
• Woo 3.1
• Woo 3.0

Specs Passing

• Woo 4.0
• Woo 3.9
• Woo 3.8
• Woo 3.7
• Woo 3.6
• Woo 3.5
• Woo 3.4
• Woo 3.3
• Woo 3.2
• Woo 3.1
• Woo 3.0

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