Adds support for custom datarefs

The following datarefs can be accessed from every other plugin within
X-Plane:
"fse/status/flying"    can be 0/1
"fse/status/leasetime" is the remaining lease time

An example to display the current flight state with a FlyWithLua script
and some screenshots are attached.
Merge pull request #2 from Teddii/master

Adds support for custom datarefs