Add Double Linestyle

[nilathedragon]

I've added double-line style to termdash.

I created a test too, to ensure a border draws properly with two lines .

[coveralls]

Pull Request Test Coverage Report for Build 599

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.238%

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Totals
Change from base Build 597:	0.0%
Covered Lines:	3947
Relevant Lines:	4423

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 599

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.238%

---

Totals
Change from base Build 597:	0.0%
Covered Lines:	3947
Relevant Lines:	4423

---

💛 - Coveralls

[mum4k]

Thanks a lot for tackling this and including a test.