Add version flag in executable

[kachick]

The version output is simple but useful for smoke tests, especially for package managers.

For example, nixpkgs has testing helpers, such as versionCheckHook and testVersion, that use this output.

I omitted the short option such as v in this PR.

After this change

> ./target/debug/scraper --version
scraper 0.24.0

> ./target/debug/scraper --help
Usage: scraper [options] SELECTOR [FILE ...]

Options:
    -H, --html          output HTML of elements
    -I, --inner-html    output inner HTML of elements
    -a, --attr ATTR     output attribute value of elements
    -c, --classes       output classes of elements
    -d, --document      parse input as HTML documents
    -f, --fragment      parse input as HTML fragments
    -i, --id            output ID of elements
    -n, --name          output name of elements
    -t, --text          output text of elements
    -h, --help          this cruft
        --install-man-page PATH
                        install real documentation
        --version       output version number

[kachick]

Thank you for merging!