Added Formula to build Dashel.

I tried here without these and the compilation was not using any optimization flags, when compiling with `brew install --verbose dashel`, for instance:
```
/usr/local/homebrew/Library/ENV/4.3/clang++   -Ddashel_EXPORTS -fPIC -I/tmp/dashel-fe5FPj/dashel-1.0.8    -o CMakeFiles/dashel.dir/dashel/dashel-common.cpp.o -c /tmp/dashel-fe5FPj/dashel-1.0.8/dashel/dashel-common.cpp
```