ssh config: include multiple files

it is useful to split up the ssh config file.

do as follows

1. create config folder $ mkdir ~/.ssh/config.d
2. create multiple config files in that folder (I group different hosts, and use a global options file)
   1. $ touch ~/.ssh/config.d/group1
   2. $ touch ~/.ssh/config.d/group2
   3. $ touch ~/.ssh/config.d/group3
   4. $ touch ~/.ssh/config.d/options
3. include the whole directory by using the standard config file

   [code language="bash" collapse="false" title="~/.ssh/config"]
   $ cat ~/.ssh/config
   Include config.d/*
   [/code]