Add configure option to tweak the location of the "preview window for hover".

[weipeng1999]

Is your feature request related to a problem? Please describe.

Now the "preview window for hover" is shown in default location (top) and it was easily cause text moving. I hope we can add some configuration to make it shown in right or bottom.

Describe the solution you'd like

May be we can add some option to tweak the "command modifiers" for previewed window. Such as:

let g:LanguageClient_PreviewWindowCommandModifiers = "botright vertical"