Add option to set title quantiles of corner/traceplot

[MJKirk]

As also suggested in https://github.com/joshspeagle/dynesty/issues/212

[joshspeagle]

This is a straightforward fix and the docstrings look good! Resolves #212.

[coveralls]

Pull Request Test Coverage Report for Build 839381565

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 71.908%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/dynesty/plotting.py	0	2	0.0%
<!--	Total:	0	2	0.0%	-->

Totals
Change from base Build 818773498:	-0.04%
Covered Lines:	3366
Relevant Lines:	4681

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💛 - Coveralls