Add support for pre/post composer commands (#67)

Author: garak

README.md

@@ -48,6 +48,19 @@ _to install dev dependencies:_
         args: analyse src/
 ```
 
+_to execute some command before and/or after composer install:_
+```diff
+      uses: docker://oskarstark/phpstan-ga
++     env:
++       PRE_COMPOSER_COMMANDS:
++         - composer config minimum-stability dev
++         - composer config prefer-stable true
++       POST_COMPOSER_COMMANDS:
++         - composer outdated
+      with:
+        args: analyse src/
+```
+
 _to skip checking the platform requirements:_
 ```diff
       uses: docker://oskarstark/phpstan-ga

entrypoint.sh

@@ -18,8 +18,21 @@ if [ "$REQUIRE_DEV" = "true" ]; then
     NO_DEV=""
 fi
 
+if [ -z "$PRE_COMPOSER_COMMANDS" ]; then
+    for str in ${PRE_COMPOSER_COMMANDS[@]}; do
+        eval "$str"
+    done
+fi
+
 COMPOSER_COMMAND="composer install --no-progress $NO_DEV $IGNORE_PLATFORM_REQS"
 echo "::group::$COMPOSER_COMMAND"
+
+if [ -z "$POST_COMPOSER_COMMANDS" ]; then
+    for str in ${POST_COMPOSER_COMMANDS[@]}; do
+        eval "$str"
+    done
+fi
+
 $COMPOSER_COMMAND
 echo "::endgroup::"
 /composer/vendor/bin/phpstan $*