cmd: simplify error message on flag parse

Related to #351.
cmd/tsuru-base: implement Flags method in AppLog command

Related to #351.
cmd/tsuru-base: use values for lines and source in AppLog from flagset

Related to #351.
cmd/tsuru-base: remove global log-related flag

Related to #351.
cmd/tsuru-base: introduce AddAppFlag function

This function add the --app flag to the given FlagSet.

Related to #351.
cmd/tsuru-base: declare --app in the AppLog flagset

Related to #351.