feat(levels): Support custom log levels

[darinspivey]

feat(levels): Support custom log levels

Some users will want to have log levels other than our defined defaults. This commit allows levels to be passed during instantiation and used with the .log() method.

• The logger will accept a custom array of "levels". All values will be upper-cased when sent to the server.
• If custom levels are provided, it will completely override the defaults. If some of those same levels are desired, they must be specified in the custom list at instantiation time.
• Validation will still be applied to the "acceptable" list of log levels, whether they're user defined or not.
• Custom log level values must be upper/lower case letters
• Convenience methods will only be added for custom levels that are also found in our defined list of defaults. Otherwise, the user must call `logger.log('my text', 'mycustomlevel').

Fixes: https://github.com/logdna/logger-node/issues/53

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chore(deps): Upgrade linting, tap and other deps

[logdnabot]

:tada: This PR is included in version 2.4.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: