Display correct Bazel release which flips a flag (bazelbuild#105)

The script will print "TBD" if the breaking release hasn't been determined yet.

Fixes bazelbuild#104

Inc flags: Display Bazel release and GitHub Issue (bazelbuild#103)

* Display the Bazel version of flags.

Bazelisk now also displays the major Bazel release in which flags are supposed to be flipped.

Required by bazelbuild/continuous-integration#869

* Print 'Bazel' as part of the version string

* Also display the Issue URL for flags

Allow using local Bazel binaries. (bazelbuild#93)

Bazel versions (e.g. in USE_BAZEL_VERSION or .bazelversion) can now
refer to absolute paths on the filesystem. As an added convenience, a
tilde prefix is expanded to the user's home directory.

Example:

```sh
$ USE_BAZEL_VERSION="~/bin/bazel-1.0" bazelisk
```

This would tell Bazelisk to not download any binaries and instead
just directly use the Bazel binary in `$HOME/bin/bazel-1.0`.

Let Bazelisk correctly separate releases and metadata from Bazel forks.

This fixes an issue where Bazelisk would accidentally reuse a cached binary or "latest releases" JSON from fork A, even when the version label refers fork B.

Also make the documentation a bit clearer and remove outdated info from it.

build.sh: Remove old binaries before building new ones.

Fix bazelbuild#61: Do not execute tools/bazel when BAZELISK_SKIP_WRAP…

…PER is set. (bazelbuild#67)

* Fix bazelbuild#61: Do not execute tools/bazel when BAZELISK_SKIP_WRAPPER is set.

Bazelisk will set this environment variable when executing Bazel (or
a wrapper found in <workspace_root>/tools/bazel). This prevents Bazelisk
from executing itself in an infinite loop, in case the wrapper is
Bazelisk or somehow executes Bazelisk.

If you're using a wrapper in <workspace_root>/tools/bazel that downloads
Bazelisk and delegates to it, please set the BAZELISK_SKIP_WRAPPER
environment variable to ensure that Bazelisk directly executes Bazel.

* Skip wrapper delegation tests on Windows

Add support for GitHub auth token (bazelbuild#62)

This adds support for BAZELISK_GITHUB_TOKEN which is a GitHub token used
for API requests when it is set. This fixes the issue of being on a
shared network and getting rate limited when using bazelisk.

Fixes bazelbuild#18

Fixed searching of workspace on Windows. (bazelbuild#56)

Support last_downstream_green Bazel binary. (bazelbuild#45)

* Support last_downstream_green Bazel binary.

This commit modifies both Bazelisk implementations to support the
"last_downstream_green" version identifier, which refers to a Bazel
binary built at the most recent commit that passed the downstream
pipeline on CI (https://buildkite.com/bazel/bazel-at-head-plus-downstream).

Part of bazelbuild/continuous-integration#583.

* Fix wrong handling of slashes.

* Slash fix, part 2.

* Add test for last_downstream_green

Get up to 100 flags (instead of 30). (bazelbuild#41)

Instead of hard-coding the limit, we could also make multiple requests,
one for each page (https://developer.github.com/v3/#pagination).

But we don't expect the number of flags to grow indefinitely, so a limit
should work.