Add --scalar flag to CLI

[mahmoud]

I was using glom in a script this week, specifically to extract single values from API calls. I noticed I kept having to trim the quotes and whitespace off of the JSON output, because even when it's a single value, it's quoted and followed by a newline.

So this feature strips the quotes and whitespace when passed a new flag: --scalar. This is only done when the glom result is not a collection. If it is, it falls back to normal json output.

Some items to consider:

1. Name?
   • I have SQLAlchemy 2.0 on the brain and scalar is everywhere. But also boltons has an is_scalar function that maps exactly to this, so even if it's jargon-y, I think it's accurate.
2. Should it be an error if the output is not a single value?
   • I think for my use case the ergonomics would be worse.
3. Right now, output is always json. Would it be better to do --output "json"|"scalar"?
   • Kinda don't want to get into the business of emitting yaml, etc.?

None of these are blockers, I am good to merge as is.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2e0c552) 98.05% compared to head (eed98dd) 98.05%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #280 +/- ## ======================================= Coverage 98.05% 98.05% ======================================= Files 27 27 Lines 4362 4371 +9 Branches 765 766 +1 ======================================= + Hits 4277 4286 +9 Misses 54 54 Partials 31 31 ```

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