adds TODO.md

Was trying to go in too many directions. This should rein it in.
converts Help into FlagSetPrintUsage
adds dotfile support TODO
roughs out ln commandline support
attempting login API request
fixes generation of API endpoint URL string
uses correct key to extract session ID
extracts asJSONBuffer()
replaces prefix "flag" with "fl" throughout
extracts TTRSSClient and its methods Call, Login

Ln.Run now issues subscribeToFeed call, but does not yet parse out the
response.
separates category TODO from feed un/sub
translates subscribeToFeed status into error message

Delivers "subscribe to feed" feature.