move defaults for config path in to config package

This allows us to determine if the user has supplied a configuration
path or not. The cli package fills in a default value so we would not
know whether the returned path is coming from the cli package defaults
or the user.
if the current config file end in .go, rename it to .json
ensure NormalizeConfig gets a directory path
let the user know when you're renaming their files
change name of the default File
rename NormalizeConfig to normalizeFilename
use constants to represent filenames
clarify error paths when json file is present

No error means the file is present. An error other than a file missing
should be returned. I'm not sure if there's a simpler way to accomplish
this but my head !Nothurts(isHurtError).
add description to exported method
replace if/else in favor of more idiomatic go
drop else and outdent block as else condition ends in a return statement
Merge pull request #92 from lcowell/json-extension

change default extension of configuration file to json