update filter search

[mzfr]

Now it's possible to search multiple values of the same type.

When I initially made change to the API it only supported filter search in the following way:

<url>?filters=attack_type:index

but failed to support something like:

<url>?filters=owners:tool owners:admin

This PR add that support

[coveralls]

Pull Request Test Coverage Report for Build 1385

• 42 of 44 (95.45%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 77.904%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tanner/api/api.py	31	33	93.94%
<!--	Total:	42	44	95.45%	-->

Totals
Change from base Build 1382:	0.1%
Covered Lines:	1576
Relevant Lines:	2023

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💛 - Coveralls

[mzfr]

@afeena what should we do about 3.6 tests failing?

[afeena]

@mzfr I think we should remove python 3.6 from travis config, what do you think?