Add support for ${compile_commands_auto}

This enbles SublimeClang to search for a suitable compile_commands.json
in the file system based on the active file.

${compile_cmds_auto} is replaced with the first directory that contains
a compile_commands.json file when traversing the file system towards the
root.
The lookup function stops looking for compile_commands.json after 25 steps.

Example using the options script available at: https://gist.github.com/3944250
"sublimeclang_options_script": "python cmake_options_script.py ${compile_cmds_auto}"