Feature/add git root keyword 102 (#103)

* feature: add a git root keyword for `--config` argument

When operating in a monorepo the new functionality of tflint (`--chdir`) breaks config file path by changing the current working directory when tflint runs.  This commit patches this gap by allowing a user to provide __GIT_ROOT__ or populate a environment variable with a custom string and when the `--config` argument is is found it replaces that keyword with the repository root (pwd in the case of pre-commit).

* feature: add documentation

README.md

@@ -145,6 +145,46 @@ repos:
         args: ["--enable require-variable-braces,deprecate-which"]
 ```
 
+## tflint Caveats
+
+### Using the `--config` argument
+
+With the introduction of `--chdir` into tflint, the `--config` argument is now bound to whatever subdirectory you are
+running the check against.  For mono-repos this isn't ideal as you may have a central configuration file you'd like to
+use.  If this matches your use-case, you can specify the placeholder `__GIT_DIR__` value in the `--config` argument 
+that will evaluate to the root of the repository you are in.
+
+```yaml
+repos:
+  - repo: https://github.com/gruntwork-io/pre-commit
+    rev: <VERSION>
+    hooks:
+    - id: tflint
+      args:
+        - "--config=__GIT_DIR__/.tflint.hcl"
+```
+
+#### Changing the placeholder value
+
+You can change the value of the placeholder by populating the `PRECOMMIT_TFLINT_REPO_ROOT_KEYWORD` environment variable.
+
+```bash
+export PRECOMMIT_TFLINT_REPO_ROOT_KEYWORD=__foo__
+
+cat <<EOF > .pre-commit-config.yaml
+---
+repos:
+  - repo: https://github.com/gruntwork-io/pre-commit
+    rev: v0.1.22
+    hooks:
+    - id: terragrunt-hclfmt
+    - id: tflint
+      args:
+        - "--config=__foo__/.tflint.hcl"
+EOF
+
+pre-commit run
+```
 
 ## License
 

hooks/tflint.sh

@@ -7,23 +7,41 @@ set -e
 # workaround to allow GitHub Desktop to work, add this (hopefully harmless) setting here.
 export PATH=$PATH:/usr/local/bin
 
-# Install any plugins defined in .tflint.hcl
-tflint --init
+# allow customization of the repo root keyword
+PRECOMMIT_TFLINT_REPO_ROOT_KEYWORD=${PRECOMMIT_TFLINT_REPO_ROOT_KEYWORD:-__GIT_ROOT__}
+
+process_arg() {
+  local arg
+  local repo_root
 
+  arg="${1}"
+  repo_root="$(pwd)"
+
+  case "${arg}" in
+    "--config"*)
+      echo "${arg//$PRECOMMIT_TFLINT_REPO_ROOT_KEYWORD/$repo_root}"
+      ;;
+    *)
+      echo "${arg}"
+  esac
+}
 
 declare -a FILES
 declare -a ARGS
 while [[ $# -gt 0 ]]
 do
   case "$1" in
-    -*) ARGS+=("$1")
+    -*) ARGS+=("$(process_arg "$1")")
       ;;
     *) FILES+=("$1")
       ;;
   esac
   shift
 done
 
+# Install any plugins defined in .tflint.hcl
+tflint "${ARGS[@]}" --init
+
 for file in "${FILES[@]}"
 do
   tflint "${ARGS[@]}" --chdir "$(dirname "$file")" --filter "$(basename "$file")"