Add global configuration file to store common parameters

[andystanton]

I almost always use the latest C++ with dexec, so rather than having to type this every time:

$ dexec -b -std=c++14 source.cpp

I'd rather just have a .dexecrc file in my user home which contained something like this:

---
c++:
    build-args:
        - -std=c++11

---

It doesn't have to be yaml, but yaml is quite succinct. Go has good out of the box support for json.