Add text substitution service

[gauthier-voron]

It is often necessary to run a command on many servers but with slightly different arguments (e.g. different id). It would be convenient to be able to define a set of key/value pairs in each server and use them as substitution in command text (much like shell variable substitution). For example:

$ silk set 1.1.1.1 id=0
$ silk set 2.2.2.2 id=1
$ silk run (1.1.1.1|2.2.2.2) run echo %{id}
1.1.1.1 :: 0
2.2.2.2 :: 1

[gauthier-voron]

Fixed by 9f9e8232ee99e9ffadf0f85f0cabf94a99417b28