Add a `get` set of subcommands

[fnichol]

As a developer,
I want a uniform way to extract full version strings or partial version coordinates,
So that I can use this information in programs, scripts, automation, etc.

Here's an example sketch of the UX…

Load, parse, validate, and print the version from a file to stdout:

$ version get file VERSION.txt

Print only the minor version coordinate to stdout:

$ version get file --minor VERSION.txt

Load, parse, validate, and print the version from ./Cargo.toml to stdout:

$ version get cargo

Same as above, but from an explicit path to a Cargo.toml:

$ version get cargo /path/to/Cargo.toml

Print only the build metadata version coordinate to stdout:

$ version get cargo --build-metadata

Parse, validate, and print only the patch coordinate from stdin to stdout:

$ echo "1.2.3" | versio get stdin --patch

As with the bump set of subcommands, there would be subcommands for:

• cargo
• file
• stdin