Support for external .dat folder

[cwi-ezoutas]

Not much as an issue but an enhancement.
Support for external .dat folders.
Concept is that the folder /repository should be intact.

So multiple dat share would be:
dat://123091857404987294873298 (/repository/MathConA/)
dat://049872874982498273483274 (/repository/MathConB/)

The created directories /repository/MathConA/.dat and /repository/MathConB/.dat are a problem since many client programs are scanning folders that are not used from the application and automatically deletes them.

So is it possible to expose where the .dat folder will be?
And create:
/datMetadata/MathConA/ (this will be the .dat for /repository/MathConA/)
/datMetadata/MathConB/ (this will be the .dat for /repository/MathConB/)

The command of the share should be:
dat share /repository/MathConA/ --external-dat /datMetadata/MathConA/
and for cloning/downloading on clients
dat clone dat://123091857404987294873298 /repository/MathConA/ --external-dat /datMetadata/MathConA/

This should be sufficient in order not to add folders and data not relevant with the data that you want to share.

[okdistribute]

This is a PR that never got finished, if someone had the spoons to try finishing it, that would be great. #193