Command-line client for https://exercism.io

To install, follow the interactive installation instructions at https://exercism.io/cli-walkthrough

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This changes how exercise metadata is stored on a user's local filesystem. Previously, after downloading an exercise a local file called .solution.json containing metatdata specific to the downloaded exercise was created under the exercises' root directory. The metadata file, meant to be hidden, caused issues for Windows users so a proposal was made to place the metadata file within a hidden directory .exercism as opposed to a hidden file. Details around the reported issue can be found here

Changes within release

• Renames the metadata file .solution.json to metadata.json
• Migrates an existing metadata file from .solution.json to .exercism/metadata.json.
• Adds a check to prevent the submission of files larger than 64KB.
• Improves messaging for various file submission errors.