Move config decoder to a separate file

[ghost]

config.go is getting rather big, it is time to split it and move decoder to it's own file.

There are no code changes here, just a simple move.

[mention-bot]

@alsamylkin, thanks for your PR! By analyzing the history of the files in this pull request, we identified @shawnburke, @anuptalwalkar and @sectioneight to be potential reviewers.

[coveralls]

Coverage remained the same at 91.365% when pulling 7706011 on configDecoder into 92659a6 on licenceFix.

[coveralls]

Coverage remained the same at 91.365% when pulling 41bcb22 on configDecoder into 92659a6 on licenceFix.

[coveralls]

Coverage remained the same at 91.174% when pulling b320422 on configDecoder into 79bffc0 on master.