Show file size in the success message

[ernilambar]

Fixes #100

[ernilambar]

@swissspidy Regarding feature tests here, should we change should be to should contain here? I would prefer to keep should be with whole output with size also. WDYT?

[ernilambar]

Related wp-cli/wp-cli#5978

[swissspidy]

Regarding feature tests here, should we change should be to should contain here? I would prefer to keep should be with whole output with size also. WDYT?

should be is probably too strict because there can be slight differences in file sizes on different environments.

I'd go for option 3: regex

Example (untested):

Then STDOUT should match /^Success: Created bar8.tar.gz \(Size: \d+ B\)$/

[davidperezgar]

Thanks for the PR!