Check for tabs instead of spaces in the doc build

This adds a tool that checks style (currently just for tabs instead of
spaces in files under include/tests/docs) and produces a travis-ci build
failure if any problems are found.

.travis.yml

@@ -38,9 +38,11 @@ matrix:
   # Documentation build:
   - os: linux
     language: docs
-    env: DOCS
+    env: DOCS STYLE
     install: pip install sphinx sphinx_rtd_theme
-    script: make -C docs html SPHINX_OPTIONS=-W
+    script:
+    - make -C docs html SPHINX_OPTIONS=-W
+    - tools/check-style.sh
 cache:
   directories:
   - $HOME/.cache/pip

tools/check-style.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+# 
+# Script to check include/test code for common pybind11 code style errors.
+# Currently just checks for tabs used instead of spaces.
+# 
+# Invoke as: tools/check-style.sh
+#
+
+found=0
+for f in `grep $'\t' include/ tests/ docs/*.rst -rl`; do
+    if [ "$found" -eq 0 ]; then
+        echo -e '\e[31m\e[01mError: found tabs instead of spaces in the following files:\e[0m'
+        found=1
+    fi
+
+    echo "    $f"
+done
+
+exit $found