Add an optional progress bar when decompressing

[bodgit]

The RVZ format knows the decompressed size so add a Size() method that
returns that value, a progress bar that implements an io.Writer and uses
that value to work out completion percentage and/or rate is
straightforward to then add.

[coveralls]

Pull Request Test Coverage Report for Build 2721243934

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 4.608%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
reader.go	0	4	0.0%

Totals
Change from base Build 2717192288:	-0.03%
Covered Lines:	20
Relevant Lines:	434

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💛 - Coveralls