Initial implementation

[rytswd]

Started the initial implementation by providing 3 commands

Available commands

• importer preview FILENAME: see what's going to happen
• importer generate FILENAME: check all Importer annotations and retrieve the referenced files
• importer purge FILENAME: remove any data between annotation pairs

Other information

• GoReleaser is used to create the CLI available
• Homebrew Tap is created separately at github.com/upsidr/homebrew-tap, which allows the CD setup here to generate the necessary Formula upgrade; I currently get 404 probably because this repo is private, but once made public, we should be able to run brew install upsidr/tap/importer
• testdata directory contains some files that are used in Go test cases

Known limitations

• File reference is done by current directory of where the CLI is run, not relative to the file. This makes it difficult to use, and should be fixed soon. Fixed, the import is done based on relative path from the file with Importer annotation
• Original implementation works with Markdown only.
• No styling such as triple-backtick or quotes, which should be provided as an option as a part of Importer annotation.
• README.md is updated using Importer, but there is no automation setup for it; we should be running Importer against all the key files such as README.md - or perhaps all the files in this repo (the performance is not great at the moment, though).

Other considerations to make

• importer preview FILENAME should give the preview of dependency graph and action plans; the current debug print should be done via --dry-run flag instead
• Some parts are using byte slice, and some using string; we should be able to stick to one - I think string should suffice most use cases
• Package structure isn't the best I found - it works, but there are too many public elements, which we should be able to tighten much more
• This is using Cobra for the CLI support, but it seems to be an overkill for something as simple as Importer. We should support flag, subcommand, and autocomplete - those are the basic features we should be able to find, and doesn't probably need full fledge Cobra support.

[rytswd]

Test code is to be added soon

[rytswd]

Although some code doesn't have the coverage, test code has been added to cover most. This is ready for review, and also I have preliminarily created a release v0.0.1-alpha for Homebrew testing based off of this branch. I will replace that once this PR is merged.

[sryoya]

LGTM except typo