Add omnifunc icase override setting

[peterstace]

This change allows a plugin user to set the icase key in omnifunc results. By setting this to 1, the user is able to filter omnifunc results in a non-case sensitive manner.

For example:

1. Enter insert mode.
2. Type fmt.
3. Trigger omnifunc completion using <C-X><C-O>
4. Deselect the first item in the list (Formatter) by pressing <C-P>.
5. If g:go_code_completion_icase is set to 0 (default), then anything you type next will filter the completion list but in a case sensitive manner. This is also the behaviour when icase is not present at all. But if g:go_code_completion_icase set to 1, then the filtering is case-insensitive.

---

My own personal workaround to get this behaviour has been to define my own omnifunc for Go files that calls go#complete#Complete and then modifies the result on the way back through.

function MyOmniFunc(findstart, base)
    let results = go#complete#Complete(a:findstart, a:base)
    if !a:findstart
        for result in results
            let result.icase = 1
        endfor
    endif
    return results
endfunction
autocmd FileType go set omnifunc=MyOmniFunc

(so if you don't want to accept this PR, there aren't any dramas on my part since I have an functional workaround).

[bhcleek]

Thanks for contributing.

LGTM

[Shane-XB-Qian]

very good, i like this !
lots langs' omni compl supported case insensitive, or lang itself did not care func name must be Upper case or not.
Unlucky, so far, golang & this plugin limited had to input 1st Upper case letter to filter omni compl items, in its most cases ...