Refactor plot styling to add a base style; set PDF truetype

[ajjackson]

In response to #24 a minimal base style is added to set Truetype.

Note that unlike Sumo we are still deferring nearly all styling to the --style option and seaborn_colorblind is just a default value.

@badw can you take a look and see if this gives satisfactory PDF files without otherwise changing the appearance?

[coveralls]

Pull Request Test Coverage Report for Build 87

• 9 of 17 (52.94%) changed or added relevant lines in 3 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 55.948%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
galore/cli/galore_plot_cs.py	0	3	0.0%
galore/cli/galore.py	3	8	37.5%

Files with Coverage Reduction	New Missed Lines	%
galore/cli/galore_plot_cs.py	1	0.0%
galore/cli/galore.py	2	26.67%

Totals
Change from base Build 85:	0.2%
Covered Lines:	428
Relevant Lines:	765

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💛 - Coveralls