retest --auto option

[AlexB52]

retest should know about few options.
One option is --auto which should be a shortcut to infer the type of ruby project used and run the appropriate command

Those two commands should be equivalent

retest 
retest --auto

Used on a rails project, it should run retest 'bundle exec rails test <test>'
Used on a rake like a gem, it should run retest 'bundle exec rake test TEST=<test>'
Used on a ruby project, it should run retest 'ruby <test>
Used on a rspec project but not using rails command, it should run retest 'bundle exec rspec <test>

As an example, aliases are used by people this way. See #15

alias rtt="if [ -x bin/rails ]; then retest 'bin/rails test <test>'; else retest 'bundle e rake test TEST=<test>'; fi"