WIP feat: Support custom theme

[ptbrowne]

With this change, it is possible to use a theme :

• From a built-in gitdocs theme (default for the moment)
• From a npm package
• From a local to the docs directory

To support this, the theme is resolved as early as possible either with syspath.resolve or with require.resolve.

I haven't completely tested this change (default works, custom works but I haven't tested the npm package way), I mainly want to check if this is the direction you want for the themes. What do you think ?

[coveralls]

Pull Request Test Coverage Report for Build 273

• 6 of 9 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 59.919%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utils/config.js	6	9	66.67%

Totals
Change from base Build 271:	-0.04%
Covered Lines:	396
Relevant Lines:	686

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 274

• 6 of 9 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 59.919%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utils/config.js	6	9	66.67%

Totals
Change from base Build 271:	-0.04%
Covered Lines:	396
Relevant Lines:	686

---

💛 - Coveralls