Add prompt mode if no words given

When reading a long English essay, It's very tedious to have to
type 'ydcv' every time you'd like to look up a word. So a prompt
would be nice when you are going to look up words extensively.
Merge pull request #9 from toctan/master

Add prompt mode to look up words interactively
space fixes for pep8
add feature of translate statement in "shell module".
Merge pull request #10 from atupal/master

add feature of translate statement in "shell module".
Merge pull request #2 from felixonmars/master

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