[v4] subshells

[colona]

Reinterpret and substitute commands in `` (or $(), or whatever) as new commands.
ex:

!wa !wa 42+1337km to newton²
!météo !wa gps coordinates Saint-Amant-sur-Fion
!startsms "bus: !ratp 131 "Les Dalias" "Porte d'Italie"" 30m

[colona]

Note: we might want two different semantics there.
Ex: !wa 1```!wa 2``, we want 2to be substitued before the1is interpreted. But for the previous!startsmsexample, we want the!ratpto be executed once the!startsms` have finished and is composing the message.

Maybe we could have something like `` for the first case and ´´ for the latter.

FUCK YOU GITHUB, lrn2markdown&typography.