Add .projections.json configuration file

Editor plugins, like @tpope's vim-projectionist, can make use of this
configuration file to enable easy jumping between "alternate" files,
among other features. For example, with this configuration and
vim-projectionist installed, you can now be editing a hook and execute
`:A` to jump to that hook's spec file. This will save some time when
working with this project.

https://github.com/tpope/vim-projectionist

Change-Id: I0500e1e6aad52e1f4da44c4ce674215a2b31f36e
Reviewed-on: http://gerrit.causes.com/45429
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: lencioni

.projections.json

@@ -0,0 +1,10 @@
+{
+  "lib/overcommit/*.rb": {
+    "alternate": "spec/overcommit/{}_spec.rb",
+    "type": "source"
+  },
+  "spec/overcommit/*_spec.rb": {
+    "alternate": "lib/overcommit/{}.rb",
+    "type": "test"
+  }
+}

CHANGELOG.md

@@ -6,6 +6,7 @@
   be `quiet` by default
 * Switch `TravisLint` pre-commit hook from deprecated `travis-lint` gem to
   `travis` gem
+* Add .projections.json configuration file
 
 ## 0.20.0