Use JSON5 parser with rc config for nxusrc

[loppear]

1. FYI, discovered that rc by default allows comments in rc json (strips before parsing) so our main goal can already be met without this PR
2. rc accepts a parser argument, so at the 'cost' of only supporting json/json5 (not ini) configs this is a simple change (if someone knows why we should continue to support ini, speak now, I don't think we've ever done that.
3. Changed ConfigurationManager to pass in the config file explicitly, turns out rc does a bunch of guessing based on cwd to find the rc file and the tests had previously been relying on --config ..... in the npm test line to force the testApp file. Application sets the config key already based on cwd, so now the tests do too.