New option: "Block" style for imported-names

[behnam]

It's a common style to have import names in individual lines, in "Block" style. It's my personal preference, as well, since it allows faster lookup with eyes and easier addition/removal of items with small diff footprint.

Example:

use freetype::freetype::{
    FT_Face,
    FT_Get_Char_Index,
    FT_GlyphSlot,
    FT_Init_FreeType,
    FT_LOAD_DEFAULT,
    FT_Library,
    FT_Load_Glyph,
    FT_New_Face,
    FT_Render_Glyph,
    FT_Render_Mode,
    FT_Set_Pixel_Sizes,
};

Looking at other options with "Block" variant, I think the name of this option should be imported_names_style or imported_names_layout. (I don't see how layout and style options are different, so please advise.)

I think I can take a shot at implementing it myself. Is there a process to approve options before getting to implementation? (I didn't see anything on this in Contributing.md.)

[behnam]

Related RFC: rust-lang/style-team#24

Duplicate issue: #788

(Closing as dup.)