Re-introduce docopt

[jdub]

Fixes #87.

• Tidy up usage string
• Switch to using bindgen::Builder directly in main()
• Builder required some new methods
• --use-msvc-mangling appears to be deprecated, and is only referenced
  in src/lib.rs. I have added a Deprecated section to the usage string
  so using the flag doesn't cause errors
• --emit-clang-ast doesn't currently disable output of Rust bindings,
  might have to do something tricky here
• Added --version based on CARGO_PKG_*
• Need to add some error handling to <input-header>

[jdub]

So,

• Is --use-msvc-mangling no longer relevant? I can see bindgen is producing #[link_name] attributes with non-gcc-looking mangling. Must be detecting the target and doing the right thing? I can junk the remaining references in this PR if you like.
• What is the appropriate behaviour for --emit-clang-ast? When passed, should it replace the Rust source entirely on stdout and --output?

Thanks. 😄

[jdub]

I'm not sure how to convince docopt to accept empty strings without an equals sign, e.g.

• --raw-line '' as used in the tests fails (it takes the next parameter, even if it's a flag, as the string)
• --raw-line='' works

@BurntSushi, suggestions?

[emilio]

Looks good, only a few nits.

It'd be nice to have the --raw-line '' working, I'll reply to the other comments now.

[emilio]

nit: If !header.is_empty()? Here and below.

[emilio]

Is --use-msvc-mangling no longer relevant? I can see bindgen is producing #[link_name] attributes with non-gcc-looking mangling. Must be detecting the target and doing the right thing? I can junk the remaining references in this PR if you like.

Yep, I think it's unused since a while ago, and it works on MSVC, see #54. Let's get rid of it.

[emilio]

What is the appropriate behaviour for --emit-clang-ast? When passed, should it replace the Rust source entirely on stdout and --output?

This flag is intended only to be a debugging aid, so making it only spit to stdout is fine.

[bors-servo]

☔ The latest upstream changes (presumably #206) made this pull request unmergeable. Please resolve the merge conflicts.

[jdub]

Interesting, @BurntSushi wants to move ripgrep to clap: BurntSushi/ripgrep#136

I'm going to let this PR cool and come back to it after the crate split discussed in #204. Then clap it. 👏

[bors-servo]

☔ The latest upstream changes (presumably #218) made this pull request unmergeable. Please resolve the merge conflicts.

[jdub]

This PR has no future.