Add option to unitary synthesis plugin interface for user config

[mtreinish]

Summary

This commit adds a new option to the unitary synthesis plugin interface
for plugins to accept free form user config. Two potential synthesis
plugins both have asked for an interface where a user can pass
configuration options through to the plugin to tune how the plugin runs.
To enable this, this commit adds a new kwarg to transpile() to pass a
configuration dictionary straight through to the plugin. As this is a
custom thing for each plugin the burden is on the plugin author to
define how this dictionary is used, implement using it, and documenting
it's use.

Details and comments

[mtreinish]

@edyounis @adekusar-drl this should add the interface you were both asking for to enable users to tweak unitary synthesis plugins. Does this work for what you need?

[coveralls]

Pull Request Test Coverage Report for Build 1446347607

• 12 of 12 (100.0%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.002%) to 82.483%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/uc.py	2	88.65%

Totals
Change from base Build 1444528657:	0.002%
Covered Lines:	49638
Relevant Lines:	60180

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💛 - Coveralls

[edyounis]

Yeah, this will work for BQSKit. Thanks for implementing it.

[adekusar-drl]

@mtreinish Thanks a lot! Yes, this is really nice to see!

[jakelishman]

Mostly looks good to me - one comment about the keyword management within the run step, but it's not 100% necessary, since I think the way it's written won't actually cause problems (at the moment, at least!).

[jakelishman]

Looks good to me!