Add code formatting check to Travis CI build

Check compliance with the code style standard established by Arduino's examples_formatter.conf Artistic Style configuration file:
https://github.com/arduino/Arduino/blob/master/build/shared/examples_formatter.conf
If the code doesn not comply with the standard, a diff between the submitted code and the auto-formatted code is shown in the log.

I did some reorganization of .travis.yml so that the formatting check was done in a single dedicated Travis CI job, rather than redundantly in every one of the jobs.

Author: per1234

.travis.yml

@@ -2,10 +2,34 @@ language: generic
 env:
   global:
     - IDE_VERSION=1.8.9
-  matrix:
-    - BOARD="arduino:samd:mkr1000"
-    - BOARD="arduino:samd:mkrwifi1010"
-    - BOARD="arduino:samd:mkrgsm1400"
+matrix:
+  include:
+    - env:
+        - BOARD="arduino:samd:mkr1000"
+    - env:
+        - BOARD="arduino:samd:mkrwifi1010"
+    - env:
+        - BOARD="arduino:samd:mkrgsm1400"
+    - env:
+        - NAME=Code Formatting Check
+      # must define an empty before_install phase, otherwise the default one is used
+      before_install: true
+      install:
+        # install Artistic Style code formatter tool: http://astyle.sourceforge.net
+        - |
+          mkdir "${HOME}/astyle";
+          wget --no-verbose --output-document="${HOME}/astyle/astyle.tar.gz" "https://iweb.dl.sourceforge.net/project/astyle/astyle/astyle%203.1/astyle_3.1_linux.tar.gz";
+          tar --extract --file="${HOME}/astyle/astyle.tar.gz" --directory="${HOME}/astyle";
+          cd "${HOME}/astyle/astyle/build/gcc";
+          make;
+          export PATH=$PWD/bin:$PATH;
+          cd "$TRAVIS_BUILD_DIR"
+        # download Arduino's Artistic Style configuration file
+        - wget --directory-prefix="${HOME}/astyle" https://raw.githubusercontent.com/arduino/Arduino/master/build/shared/examples_formatter.conf
+      script:
+        # check code formatting
+        - find . -regextype posix-extended -path './.git' -prune -or \( -iregex '.*\.((ino)|(h)|(hpp)|(hh)|(hxx)|(h\+\+)|(cpp)|(cc)|(cxx)|(c\+\+)|(cp)|(c)|(ipp)|(ii)|(ixx)|(inl)|(tpp)|(txx)|(tpl))$' -and -type f \) -print0 | xargs -0 -L1 bash -c 'if ! diff $0 <(astyle --options=${HOME}/astyle/examples_formatter.conf --dry-run < $0); then echo "Non-compliant code formatting in $0"; false; fi'
+# default phases
 before_install:
   - wget http://downloads.arduino.cc/arduino-$IDE_VERSION-linux64.tar.xz
   - tar xf arduino-$IDE_VERSION-linux64.tar.xz