Support for multipart multipart/form-data and file upload

Support multipart data and uploading file as a form parameter in POST and PUT methods.
There can be a folder that contains all the files that will be uploaded while testing, this path can be set as an argument while running silk.
The names of the file mentioned in the markdown could be relative to this folder. We can add a comment after the field that the filed is a file field and it's value is a file path.

```
name: Mat
CV: CV/mat-cv.pdf // file
```

No source files, tests, or entry points are named in the issue. Start by tracing how Silk parses markdown form parameters and builds POST and PUT requests; the work is done when a configured upload folder resolves relative file paths and sends multipart form fields and files using the proposed comment syntax.

Written by the indexing model from the issue text.