Add verbs to lowercase/uppercase keys

[johnkerl]

Nice! If there were also a verb to change the case of keys (e.g. lowercase, uppercase), fields could be normalized across different files without having to use the nuclear $ = {} option in a put expression 👍

Originally posted by @archetyped in #1167 (comment)

[johnkerl]

@archetyped #1217

[archetyped]

Nice! Do you think it would be more flexible or easier to manage if all case manipulation was under a single verb (e.g. case or change-case) with transformations passed as options (e.g. -l for lowercase, -u for uppercase, etc.)?

This allows for additional transformations (title case, sentence case, etc.) to all live under a single verb, rather than expanding the verb-space.

Certain case transformations are particularly useful when working with values (e.g. title case), so an option to specify fields for the verb to act on (like unsparsify) would be great.

[johnkerl]

On #1215:

• mlr upcase is mlr case -u
• mlr downcase is mlr case -l
• New options -s and -t for sentence-case and title-case
• New -f to specify field names to be operated on (default all)

Note the first two bullets are backward-incompatible with head but not with any previously released Miller version.

[johnkerl]

@archetyped I believe #1215 resolves this -- if not please let me know and we can re-open -- thanks!

[archetyped]

@johnkerl looks great, thanks!