Proposal: Improve compatibility with JSON in configuration

[wallyqs]

The configuration format from the server is almost already a superset of JSON with exception of the following two cases:

1. trailing commas from values at the top level are not allowed(although they are supported inside of maps)

2. initial and final brackets at the top level section not supported

This change relaxes the two cases above to just skip those tokens so that the config syntax can also support valid JSON objects for configuring the server which might be helpful when generating it programmatically.

• Link to issue, e.g. Resolves #NNN
• Documentation added (if applicable)
• Tests added
• Branch rebased on top of current master (git pull --rebase origin master)
• Changes squashed to a single commit (described here)
• Build is green in Travis CI
• You have certified that the contribution is your original work and that you license the work to the project under the Apache 2 license

Changes proposed in this pull request:

• Skip initial and final brackets from config syntax
• Ignore trailing commas at the top level

/cc @nats-io/core

[coveralls]

Coverage increased (+0.03%) to 92.184% when pulling c45d2f5 on wallyqs:conf-json-compat into b2a9ed9 on nats-io:master.

[derekcollison]

LGTM