Adding various opt-setting helpers

[dgkf]

Closes #8 Closes #9

• Adds various utility functions for modifying options: opt_set(), opt()<- and opt_set_local().

• Trying to retrieve an option that is not yet defined will now default to throwing a warning. This behavior can be modified using the on_missing argument to functions that fetch option values.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (a2c1102) 89.34% compared to head (6315833) 90.57%.

Files	Patch %	Lines
R/options_get.R	96.66%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10 +/- ## ========================================== + Coverage 89.34% 90.57% +1.22% ========================================== Files 10 10 Lines 413 456 +43 ========================================== + Hits 369 413 +44 + Misses 44 43 -1 ```

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