This commit also purges .gitignore, since the files mentioned in it
should be handled by global ignore file.

Fixes #4378.

This cleans up the Alacritty scripts a bit by removing some of them
which are not recommended to be used anymore and switching from the
official FlameGraph tool to the more specialized Rust FlameGraph
implementation.

Following Rust's standard, the lockfiles for Alacritty's
sub-libraries have been removed.

One instance of the deprecated `ATOMIC_USIZE_INIT` has
also been removed.

This fixes #2040.

Initial support for Windows is implemented using the winpty translation
layer. Clipboard support for Windows is provided through the `clipboard`
crate, and font rasterization is provided by RustType.

The tty.rs file has been split into OS-specific files to separate
standard pty handling from the winpty implementation.

Several binary components are fetched via build script on windows
including libclang and winpty. These could be integrated more directly
in the future either by building those dependencies as part of the
Alacritty build process or by leveraging git lfs to store the artifacts.

Fixes #28.

Snapcraft makes Linux packaging very simple in a cross-distro
way. This adds the snapcraft.yaml file to setup a snap of
alacritty.

Fix tests and add line select

Refactor BidirectionalIter to remove if blocks

Allow for cells tagged with WRAPLINE to continue expanding the selection

Reorganize config into structs

Add test coverage that callbacks are called

Cleanup mouse config

- Uses Duration type for ClickHandler::threshold
- Removes `action` property from ClickHandler--this can be added in a
  backwards compatible way later on
- Renames ClickState::DblClick to DoubleClick

fixup! Cleanup mouse config

This uses the rustc-test crate, a copy of the standard test crate, to
dynamically create tests for each reference test. No need to remember to
update the macro, just add the directory to ref!

Resolves #8