Add global configuration file to store common parameters

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

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

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

``` yaml

---
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.
