add writing to log file when logging + --read-log | -rl switch

[saltkid]

1. write to log file as well instead of just printing to console

2. --read-log | -rl

values: <log-header>, no value
default if no value: time

read log file based on header passed. it will read the latest log in the logs directory.

proposal: accept two args: path then <log-header>

this will need changes in Arg as a whole which will affect all commands, switches, and flags

update README and wiki too