Previous/current planet/system conditions and substitutions (#225)

Author: Amazinite

wiki/CreatingMissions.md

@@ -243,6 +243,10 @@ Certain characteristics of a mission, such as the cargo or the destination plane
 * `<model>` = the display model of your flagship, or the display model of an NPC if used in text that is created by an NPC interaction **(v. 0.10.9)**
 * `<flagship>` = the name of your flagship in all scenarios **(v. 0.10.14)**
 * `<flagship model>` = the display model of your flagship in all scenarios **(v. 0.10.14)**
+* `<current planet>` = the planet that the player is currently landed on **(v. 0.11.1)**
+* `<current system>` = the system that the player is currently in **(v. 0.11.1)**
+* `<previous planet>` = the planet that the player was landed on prior to the current one **(v. 0.11.1)**
+* `<previous system>` = the system the player was in prior to the current one **(v. 0.11.1)**
 * `<start planet>` = the display name of the planet you started on **(v. 0.11.1)**
 * `<start system>` = the display name of the system containing the planet you started on **(v. 0.11.1)**
 * `<start date>` = the date you started the game, in a short form **(v. 0.11.1)**