Switch from Docopt to Clap.

There were two important reasons for the switch:

1. Performance. Docopt does poorly when the argv becomes large, which is
   a reasonable common use case for search tools. (e.g., use with xargs)
2. Better failure modes. Clap knows a lot more about how a particular
   argv might be invalid, and can therefore provide much clearer error
   messages.

While both were important, (1) made it urgent.

Note that since Clap requires at least Rust 1.11, this will in turn
increase the minimum Rust version supported by ripgrep from Rust 1.9 to
Rust 1.11. It is therefore a breaking change, so the soonest release of
ripgrep with Clap will have to be 0.3.

There is also at least one subtle breaking change in real usage.
Previous to this commit, this used to work:

    rg -e -foo

Where this would cause ripgrep to search for the string `-foo`. Clap
currently has problems supporting this use case
(see: https://github.com/kbknapp/clap-rs/issues/742),
but it can be worked around by using this instead:

    rg -e [-]foo

or even

    rg [-]foo

and this still works:

    rg -- -foo

This commit also adds Bash, Fish and PowerShell completion files to the
release, fixes a bug that prevented ripgrep from working on file
paths containing invalid UTF-8 and shows short descriptions in the
output of `-h` but longer descriptions in the output of `--help`.

Fixes #136, Fixes #189, Fixes #210, Fixes #230

.travis.yml

@@ -30,13 +30,10 @@ matrix:
       env: TARGET=x86_64-apple-darwin
     # Minimum Rust supported channel.
     - os: linux
-      rust: 1.9.0
-      env: TARGET=x86_64-unknown-linux-musl
-    - os: linux
-      rust: 1.9.0
+      rust: 1.11.0
       env: TARGET=x86_64-unknown-linux-gnu
     - os: osx
-      rust: 1.9.0
+      rust: 1.11.0
       env: TARGET=x86_64-apple-darwin
 
 before_install: