build: add custom IDEA project icon

Icon for IntelliJ IDEA placed into .idea,
so the project is easier to pick out in the IDE.

Author: spannm

.gitignore

@@ -9,7 +9,8 @@
 **/NOTES.txt
 *.iml
 .checkstyle
-.idea
+.idea/*
+!.idea/icon.svg
 .project
 .~lock*
 *.kate-swp