EliteDangerous: added jumpTarget stuff

You can now set a jump target. This will start writing the distance to
said target to `>jumpTargetFile` for inclusion e.g. in OBS.

If SpanshAttack currently has a target system, jump target will default
to that. A manually set target will take precedence.

docs/EliteDangerous.md

@@ -241,6 +241,7 @@ The commands in here do random more or less useful things.
 
 * `bind keys;reset key binds`: Reloads your key binds through the bindED plugin. 
   You should do that after changing anything in the controls options.
+* `clear jump target`: Clears the current jump target.
 * `copy current system`: Copies the current system name into the clipboard.
 * `distance [from;to] […]`: Tells you the distance from your current position to 
   the other thing you mentioned and is supported in the command. (requires 
@@ -259,6 +260,8 @@ The commands in here do random more or less useful things.
 * `open EDDI options; configure EDDI`: Opens the EDDI configuration window.
 * `open [the;] miner’s tool`: Opens https://edtools.ddns.net/miner in your 
   default browser.
+* `set jump target`: Sets the jump target to the currently targeted system.  
+  Distance will be written to the configured file.
 * `[start;stop] [EDISON;navigation]`: Hits `CTRL+ALT+E` which just so happens to 
   be the start/stop hotkey I have set in E.D.I.S.O.N.
 * `[what’s;what is] left to [map;be mapped]`: Tells you which bodies EDDI thinks 
@@ -366,6 +369,8 @@ if you have included EliteDangerous.
   side. Default: false.
 * `>flightAssistOff` (boolean): whether to automatically toggle FlightAssist off 
   on liftoff. Default: true.
+* `>jumpTargetFile` (string): the file the distance to the currently set jump 
+  target will be written to.
 * `>targetSubsystem` (string): the default target subsystem. Unused. Default: 
   “drive”.
 * `python.ScriptPath` (string): the path you have placed the compiled python