Add shell completions

[Freed-Wu]

Rewrite of #45

gscholar --print-completion bash | sudo tee /usr/share/bash-completion/completions/gscholar
gscholar --print-completion zsh | sudo tee /usr/share/zsh/site-functions/_gscholar
gscholar --print-completion tcsh | sudo tee /etc/profile.d/gscholar.completion.csh

❯ gscholar -<TAB>
option
--all                show all bibtex results
-a                   show all bibtex results
--debug              show debugging output
-d                   show debugging output
-f                   Output format. Available formats are: bibtex, endnote, refman,wenxianwang [default: %(default)s]
--help               show this help message and exit
-h                   show this help message and exit
--outputformat       Output format. Available formats are: bibtex, endnote, refman,wenxianwang [default: %(default)s]
--print-completion   print shell completion script
--rename             rename file
-r                   rename file
-s                   Page number to start parsing PDF file at.
--startpage          Page number to start parsing PDF file at.
--version            show program's version number and exit

And fix bug about %(prog)s

[Freed-Wu]

@venthur How about this PR?