Support custom path-mapped dependencies

[weijiekoh]

This PR allows circom to recognise custom path-mapped dependencies so this works:

circom ./foo.circom -a @customlib=./path/to/customlib/

include "@customlib/bar.circom";

This benefits developers who publish circom files to NPM. The way node_modules is laid out prevents a circom file in one module from correctly importing a circom file in another module. By specifying custom path-maps, developers can sidestep this problem.

It includes a simple Mocha test.