Fix error where usageParams were being set incorrectly

[AndrewBresee]

Observed:
When running contentful-export against a space with a larger than max size, the 400 error "Response size too big, was: x. Maximum allowed response size: 7340032b." is thrown. This happens even when maxAllowedLimit is set in options.

Reason:
This happens because runContentfulExport does not handle usageParams correctly. Inside of the runContentfulExport function, let {opts, errorLogFile} = usageParams is set. ES6 interprets this to mean opts = usageParams.ops and usageParams does not have the property opts on it. usageParams comes in exactly as it is entered into options.

Fix:
let opts = usageParams should be set first. This will fix the issue setting maxAllowedLimit and includeDrafts. However, errorLogFile is still undefined.

Next Steps:
errorLogFile is not used when calling runContentfulExport within contentful-export/bin/contentful-export.

[AndrewBresee]

opts.sourceSpace = opts.sourceSpace || usageParams.spaceId;
 opts.sourceManagementToken = opts.sourceManagementToken || usageParams.managementToken;

needed to be added because const clients = createClients(opts) was not handling opts correctly. if (!opts) was always being hit, so createClients was always being passed opts with sourceSpace and sourceManagementToken.

[AndrewBresee]

exportToFile = true needs to be handled properly. Will push changes in a moment.

[axe312ger]

@AndrewBresee I pushed a fix to the readme describing the correct usage, will be in master soon.

Could you fix your issues with the comment of @Khaledgarbaya?

If yes, feel free to close this one. Otherwise lets escalate here :)