[fix]: Cannot use config file without json extension

[carlosjeurissen]

Config files without a json extension, for example ".posthtmlrc" can't be used. This is due to the use of require to fetch the config file.

Please consider replacing require with:
JSON.parse(fs.readFileSync(argv.config, 'utf8'))

Talking of which, the config file gets fetched once for each file you process. To solve this, please consider fetching the config file only once, like this:

var cfgData = argv.config && JSON.parse(fs.readFileSync(argv.config, 'utf8'));

and use it in the processing file like this:
var cfg = cfgData ? Object.keys(cfgData) : [];
and
if (cfgData) {config = Object.assign(cfgData, config)}

[Scrum]

@carlosjeurissen HI, hanks, I'll think, it may be PR ?