Adds support for different log levels.

[rafagil]

Hi!
This PR intends to solve the issue #201

This adds different log levels to the lib (WARNING and ERROR):

If WARNING is set, it will show everything, but if ERROR is set, warnings won't be shown in the console.

How to Use

You can set the log level either on the options object or globally using:

domtoimage.impl.options.logLevel = domtoimage.logLevels.ERROR

Possible improvements

• Other levels could be implemented (INFO, DEBUG) but I didn't in this PR because its not being used.
• The copyOptions function could be way smaller if rewritten, but I opted to keep the current coding style.