Reject invalid Description strings

[jwodder]

GitHub requires label descriptions to be no more than 100 characters long. labelmaker should reject descriptions that do not satisfy this.

It's likely that GitHub enforces further checks, such as prohibiting newlines and/or control characters. Validate for these as well.

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Observations:

• Like label names, descriptions are normalized by having leading & trailing SP, LF, CR, TAB, FF, and VT character stripped, and internal LFs are converted to SP.
• Leading & trailing NULs are also stripped from descriptions.
• Unlike label names, there do not appear to be any restrictions on what characters are allowed in descriptions other than LF.

[jwodder]

Decision: Like #4, labelmaker will match GitHub's description normalization but not prohibit too-long descriptions.