Plain authorization support

[RiviqVH]

Describe your changes

Add support for plain authorization & fix minor issue with requirements on Windows.

Internal Jira ticket number or external issue link

Issue #140

Testing procedure/screenshots(if appropriate):

Tested connection to Impala server using plain authorization, which worked. I can provide screenshots upon request.

Checklist before requesting a review

• I have performed a self-review of my code
• I have formatted my added/modified code to follow pep-8 standards
• I have checked suggestions from python linter to make sure code is of good quality.

[TapasSenapati]

Thanks for the initiative. Can you attach some screenshots showing a successful dbt debug connection.

[RiviqVH]

@TapasSenapati Sure! I blotted out some potentially sensitive user info, but here it is: