Make erlexec an optional dependency

[alappe]

While the option to use the node compiler via erlexec is very nice, it could be an optional dependency for those who want it.

[coveralls]

Pull Request Test Coverage Report for Build fcc0a6d29f0b94b2dd1ddd18f4474e37e0283b46-PR-9

• 4 of 22 (18.18%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-15.4%) to 81.197%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/compilers/node.ex	4	22	18.18%
<!--	Total:	4	22	18.18%	-->

Totals
Change from base Build 5d3156fd56aa87ee8c00e6230ba037195b9825b5:	-15.4%
Covered Lines:	95
Relevant Lines:	117

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💛 - Coveralls

[akoutmos]

Sorry about the delay! I merged in PR #10 with your work along with the fix for the tests and an error message to let people know how to use the node compiler instead. Thanks for the contribution! Cutting a new release in the next 10 minutes :)