Implemented dynamic compilation options.

By entering a shell command surrounded by backticks `` ` `` in the **.clang_complete** file, clang complete will execute command, and treat any output to stdout as more lines in the **.clang_complete** file.

This is especially helpful for interfacing with build systems which automatically provide dependencies.

For example:

**.clang_complete**

    -Ihelpers.h
    `tmake --get-compilation-flags`

This would include **helpers.h** as well as process the output of the make system.

There may be security concerns with this patch as it allows for code execution, but in my opinion it is a useful feature.