Fix suggest query names for Completion suggestor. (Finish work on #1098)

[Caiofcas]

Basically a rebase of #1098. From what I understand it also closes #1650 (that is how I found the old PR).

If anything has changed in the code style from then I can adequate the work already done by tell-k, but it seems a small enough change.

Original PR description:

According to official documentation, Completion Suggester seems to be using correct prefix rather than text.

POST music/_search?pretty
{
    "suggest": {
        "song-suggest" : {
            "prefix" : "nir", 
            "completion" : { 
                "field" : "suggest" 
            }
        }
    }
}

According to official documentation, Completion Suggester seems to be using correct prefix rather than text.

POST music/_search?pretty
{
    "suggest": {
        "song-suggest" : {
            "prefix" : "nir", 
            "completion" : { 
                "field" : "suggest" 
            }
        }
    }
}

In the Search.suggest method, however, it always becomes text, so I fixed it.

In Completion Suggestor you can also specify regex.

POST music/_search?pretty
{
    "suggest": {
        "song-suggest" : {
            "regex" : "n[ever|i]r",
            "completion" : {
                "field" : "suggest"
            }
        }
    }
}

I also modified it so that the user can select it.

# Sample of use

# Term Suggester
s = s.suggest('song-suggest', 'nir', term={'field': 'suggest'})

# Completion Suggester + prefix query
s = s.suggest('song-suggest', 'nir', completion={'field': 'suggest'})

# Completion Suggester + regex query
s = s.suggest('song-suggest', regex='n[ever|i]r', completion={'field': 'suggest'})
# "regex"  must be keyword argument.

Please feel free to merge it. thx.

[miguelgrinberg]

LGTM. Thanks!