Add .fish extension to fish shell

If a user hits Ctrl-C to exit out of a search in the middle of printing
a line, we don't want to leave the terminal colors screwed up for them.
Catch Ctrl-C using the ctrlc crate, obtain a stdout lock to ensure that
other threads don't continue writing after we do so, reset the terminal,
and exit the program.

Closes #119

Move all gitignore matching to separate crate.

This PR introduces a new sub-crate, `ignore`, which primarily provides a
fast recursive directory iterator that respects ignore files like
gitignore and other configurable filtering rules based on globs or even
file types.

This results in a substantial source of complexity moved out of ripgrep's
core and into a reusable component that others can now (hopefully)
benefit from.

While much of the ignore code carried over from ripgrep's core, a
substantial portion of it was rewritten with the following goals in
mind:

1. Reuse matchers built from gitignore files across directory iteration.
2. Design the matcher data structure to be amenable for parallelizing
   directory iteration. (Indeed, writing the parallel iterator is the
   next step.)

Fixes #9, #44, #45

update python types to include Cython files

Add zsh type

Mention Nix package in README

In the `README.md` document, where said document documents the
availability of pre-built packages of ripgrep, document the
availability of such a package from the package management system Nix.

Use svg for travis badge

Add instructions for installation on Fedora 24+ and RHEL/CentOS 7.

This particular bug was triggered whenever a search was run in a directory
with a parent directory that contains a relevant .gitignore file. In
particular, before matching against a parent directory's gitignore rules,
a path's leading `./` was not stripped, which results in errant matching.

We now make sure `./` is stripped.

Fixes #184.

Please note that the referenced copr repository should just be a temporary home while the Fedora/EPEL package review is pending.

https://bugzilla.redhat.com/show_bug.cgi?id=1380442

add .tcl file extension

Add `handlebars` type