Provide 1 .hpp + 1 .cpp distribution of Catch2

This commits also adds a script that does the amalgamation of headers
and .cpp files into the distributable version, removes the old
`generateSingleHeader` script, and also adds a very simple compilation
test for the amalgamated distribution.

.travis.yml

@@ -174,6 +174,12 @@ matrix:
 before_script:
   - export CXX=${COMPILER}
   - cd ${TRAVIS_BUILD_DIR}
+  # We want to regenerate the amalgamated header if the extra tests
+  # are enabled.
+  - |
+    if [[ ${EXTRAS} -eq 1 ]]; then
+      python3 ./tools/scripts/generateAmalgamatedFiles.py
+    fi
 
   - |
     if [[ ${CPP17} -eq 1 ]]; then