Infer HTTP method if not specified

[ducaale]

This PR adds support for omitting the HTTP method in which case it will default to GET if the body is empty or to POST otherwise.

http :8080/users               # will be translated to `http get :8080/users`
http :8080/users name=ali      # will be translated to `http post :8080/users name=ali`
http put :8080/users name=ali  # it is still possible to specify the HTTP method

This is a hacky and temporary solution to issue #2. Here are the drawbacks I can think of so far:

1. The Method and the URL are now treated as one in the manpages

Before

USAGE:
    ht [FLAGS] [OPTIONS] <METHOD> <URL> [REQUEST_ITEM]...
ARGS:
    <METHOD>             The HTTP method to be used for the request [possible values: GET, POST, PUT, PATCH, DELETE]
    <URL>
    <REQUEST_ITEM>...    Optional key-value pairs to be included in the request

After

USAGE:
    ht.exe [FLAGS] [OPTIONS] <[METHOD] URL> [REQUEST_ITEM]...

ARGS:
    <[METHOD] URL>
    <REQUEST_ITEM>...    Optional key-value pairs to be included in the request

2. It is not possible to have a flag between the Method and the URL. For example, the following will not be a valid usage anymore

ht post --offline httpbin.org/post name=ahmed

Instead, any flags should be before the Method or after the URL

ht --offline post httpbin.org/json name=ahmed
ht post httpbin.org/json --offline name=ahmed

Closes #2