feat(proxy): allow contextless proxy configuration

[chimurai]

Create proxy without context parameter; using server's path config instead:

app.use('/api', proxy('http://localhost:3000', {changeOrigin:true}));

app.use('/api', proxy({target: 'http://localhost:3000', changeOrigin:true}));

[coveralls]

Coverage increased (+0.01%) to 99.313% when pulling 985e867 on contextless-configuration into aa89e1f on master.

[coveralls]

Coverage increased (+0.007%) to 99.31% when pulling 331c8fa on contextless-configuration into aa89e1f on master.

[coveralls]

Coverage increased (+0.007%) to 99.31% when pulling 331c8fa on contextless-configuration into aa89e1f on master.