Use psalm for static analysis

Don't check the main Parsedown file just yet

.travis.yml

@@ -26,7 +26,7 @@ matrix:
 
 install:
   # remove packages with PHP requirements higher than 7.0 to prevent composer trying to resolve these, see: https://github.com/composer/composer/issues/6011
-  - composer remove friendsofphp/php-cs-fixer --dev --no-update --no-interaction
+  - composer remove vimeo/psalm friendsofphp/php-cs-fixer --dev --no-update --no-interaction
   - composer install --prefer-dist --no-interaction --no-progress
 
 script: composer test-units
@@ -42,6 +42,7 @@ jobs:
       install: composer install --prefer-dist --no-interaction --no-progress
       script:
         - '[ -z "$TRAVIS_TAG" ] || [ "$TRAVIS_TAG" == "$(php -r "require(\"vendor/autoload.php\"); echo Erusev\Parsedown\Parsedown::version;")" ]'
+        - composer test-static
         - composer test-formatting
 
     - stage: Test CommonMark (weak)

composer.json

@@ -18,6 +18,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^7.4|^6.5.13|^5.7.27|^4.8.36",
+        "vimeo/psalm": "^3.0",
         "friendsofphp/php-cs-fixer": "^2.13"
     },
     "autoload": {
@@ -28,9 +29,12 @@
     },
     "scripts": {
         "test": [
+            "@test-static",
             "@test-units",
             "@test-formatting"
         ],
+        "test-static": "vendor/bin/psalm",
+        "test-dead-code": "vendor/bin/psalm --find-dead-code",
         "test-units": "vendor/bin/phpunit",
         "test-commonmark-weak": "vendor/bin/phpunit tests/CommonMarkTestWeak.php",
         "test-formatting": "@composer fix -- --dry-run",

psalm.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0"?>
+<psalm
+    totallyTyped="true"
+    strictBinaryOperands="true"
+    checkForThrowsDocblock="true"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+          <file name="src/Parsedown.php" />
+      </ignoreFiles>
+    </projectFiles>
+</psalm>