add configuration checker

[lsloan]

Add a mode to the application in which it reads .env and checks the configuration, then exits without doing any migrations. This would let the user get some validation for the values in the file. Things that should be checked…

• Canvas API — Check by connecting to the Canvas API and performing a simple query using the supplied URL and key. (Could be the tool ID queries, below.)
• UDW — Check by connecting to UDW and performing a simple query.
• Account ID — Check by looking up the ID in the Canvas API and displaying its name.
• Term IDs — Check by looking up the IDs in the Canvas API and displaying their names and dates.
• Tool IDs — Check by looking up the IDs in the Canvas API and displaying their names.

[lsloan]

This is partially resolved by PR #32 for #22. To completely implement this feature, we'd need…

1. Another parameter (probably Boolean) that indicates whether the run is for config check only.
2. Simple connection checks with Canvas API and UDW.