Ignore cache files and files in bin other than the sample

Note that this doesn't ignore the `repos' directory that it currently creates.
This is under the assumption that the directory structure will be changed to
what is described in:

zplug/zplug#71 (comment)

Author: NigoroJr

.gitignore

@@ -1,8 +1,9 @@
 .DS_Store
 **/.DS_Store
 .ignore
-bin/zspec
+bin/*
+!bin/zplug-env
 **/zcompdump
 **/*~
 **/*.swp
-test/_fixtures/.cache
+.cache