blamer: Theme blamer-avatar-folder

[leungbk]

blamer-avatar-folder stores avatars corresponding to GitHub or GitLab
contributors.

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Presently, the package defines only a single variable corresponding to a data/config directory, but the name does not imply that it precludes the later introduction of other such defcustoms, so I followed rule 2.3.5:

If a package only defines a single variable that specifies a data (or config) directory, then the directory name should nevertheless be just the package name. E.g., the path used for sx-cache-directory from the sx package is sx/cache/, not sx-cache/.

[tarsius]

Thanks!