Added support for passing custom s3 client

[creatorrr]

No description provided.

[mpenkov]

@creatorrr Thank you for making the pull request. While in general the change looks OK, I have a few questions:

• What is the use case for passing a custom S3 client?
• Is there a benefit to including this as a parameter, as opposed to mocking it?
• I notice that the top-level interface has not changed. How does the user pass this new parameter?

[menshikh-iv]

Ping @creatorrr

[yganti87]

Hello @mpenkov - One case I ran into where a custom s3 client needed to be passed was for cross account access using STS. I couldn't find a way of doing this in the current version.

[eschwartz]

Looks like this PR has gone stale, but I also need this functionality (see #199)
Give me a couple days, but I'm going to see if I can get this finished up.

[piskvorky]

Fixed by #201.