Doesn't depend on all database backends.

[hugopl]

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This works by having a wrapper that compiles the CLI on first use, when the database backend libraries were already downloaded by shards.

Changes:

• Rename src/micrate-bin.cr to src/micrate-cli.cr
• Add src/micrate-wrapper.cr that compiles in a post install to bin/micrate.
• bin/micrate compiles src/micrate-cli to bin/micrate-cli-<VERSION>
• src/micrate-cli.cr only require database backends found in lib/
• Do not pin crystal-db version to avoid having to release new micrate versions on every crystal-db release.

Fix #87

[!CAUTION] Despite of the code dealing with windows I only tested this on Linux, would be nice if someone else test this (and possible fix it) on windows before merge.

[hugopl]

@crimson-knight could you review/provide some feedback on this?