Pretty print JSON config.

[Tonkpils]

Also, print line number highlight the entire JSON line when a syntax error occurs

errors now point to the line number

2015/04/03 13:17:37 error parsing JSON in the config file /Users/tonkpils/.exercism.json:
invalid JSON syntax at line 3:
    3:  "dir": "/Users/tonkpil\s <~
invalid character 's' in string escape code

And the file now looks like

$ cat ~/.exercism.json                                                                                
{
    "apiKey": <api_key>,
    "dir": "/Users/tonkpils/exercism",
    "api": "http://exercism.io",
    "xapi": "http://x.exercism.io"
}

[kytrinyx]

Sweet!