Added save_xxx snippets

Saving some values in temprary variables is common process in writing
Vim script.  I made snippets for them for DRY.

Added save_xxx snippets
- save_pos
- save_register
- save_options

Followings are examples of them.

    let pos_save = getpos('.')

    call setpos('.', pos_save)

    let save_reg_g = getreg('g')
    let save_regtype_g = getregtype('g')

    call setreg('g', save_reg_g, save_regtype_g)

    let selection_save = &selection
    let &selection =

    let &selection = selection_save
Merge pull request #183 from rhysd/add/save_xxx_snippets

Added save_xxx snippets