fix count/range propagation through fallback menu

prevent endless loop in shortcut menu on 2nd+ run

expand <Leader> and <LocalLeader> in shortcut menu

This allows for more reliable fallback behavior, where the user's input
would match shortcuts containing their <Leader> and <LocalLeader> keys.

fzf: prefer user's literal input when tie-breaking

FZF's default tie-breaking configuration prioritized results that had
a different composition of cases than my fully lowercase input ("eg"):

    <Leader>e            go to end of current word in camel/snake case
    <Space>eg            (fzf) go to file in git repository
    <Space>eG            (fzf) go to file in git status
  > <Space>Eg            git revert buffer
    203/367
  > eg

This problem is solved by passing the --tiebreak=begin option to FZF:

    <Leader>e            go to end of current word in camel/snake case
    <Space>eG            (fzf) go to file in git status
    <Space>Eg            git revert buffer
  > <Space>eg            (fzf) go to file in git repository
    203/367
  > eg

Notice how the first match now reflects my fully lowercase input ("eg").

use a decorator to inject typeahead input into FZF

Vim workaround to pass unmatched typeahead to FZF

junegunn/fzf.vim#307

Thanks to Junegunn Choi for sharing this solution.

README: document discovery and fallback shortcuts

README: move screencast out of master to gh-pages