Add how-to about running the tour offline

[wilzbach]

As in the go tour, we could have a short page or subpage in the first pages that explains how to run the tour via dub.

Follow-up to #250

[stonemaster]

Dependency: #137

[wilzbach]

@stonemaster afaict you need to release new git versions once you fixed something - dub will download the latest version automatically, not the latest git version.
There is also dub fetch dlang-tour --version="~master", but I am not sure whether we should recommend this command.

[wilzbach]

Just bumping this. Basically we only need to decide whether we advertise the use of --version="~master" (imho it does make sense because for now it saves us a release process and our latest, greatest content is on "master").

... and then put the commands on the welcome pages:

dub fetch dlang-tour  --version="~master"
dub run dlang-tour

[wilzbach]

This issue was moved to dlang-tour/english#55