Support optional sense-like parameters for method/url specifiation

Allows specifying a `.es` file with the following:

```
POST /_search?pretty
{
  "query": {
    "match_all": {}
  }
}
```

Hitting `C-c C-c` anywhere inside the `{ ... }` executes the request, or a region if one is currently selected. `C-c C-a` forces the region-or-whole-buffer execution, prompting for URL (as in previous behavior).

Relates to #4