Add support for bazel (#56)

* Add support for bazel

Bazel support for the magic_enum repository would help others using
bazel integrate this library into their projects.

This PR adds a BUILD file which enables magic_enum to be imported
seamlessly into other WORKSPACE files without having to write a custom
BUILD file.

It also adds bazel binaries & tests to enable running them via bazel
locally:

   export CC=clang ; bazel build //... ; bazel test //... ; bazel run //:example

Finally, bazel compilation artifacts are added to the gitignore file.

* Update BUILD file & specify http_archive in README

* Lint BUILD file

BUILD.bazel

@@ -0,0 +1,63 @@
+licenses(["notice"])
+
+exports_files(["LICENSE"])
+
+package(default_visibility = ["//visibility:public"])
+
+cc_library(
+    name = "magic_enum",
+    hdrs = ["include/magic_enum.hpp"],
+    includes = ["include"],
+)
+
+cc_binary(
+    name = "example",
+    srcs = ["example/example.cpp"],
+    copts = ["-std=c++17"],
+    deps = [":magic_enum"],
+)
+
+cc_binary(
+    name = "enum_flag_example",
+    srcs = ["example/enum_flag_example.cpp"],
+    copts = ["-std=c++17"],
+    deps = [":magic_enum"],
+)
+
+cc_binary(
+    name = "example_custom_name",
+    srcs = ["example/example_custom_name.cpp"],
+    copts = ["-std=c++17"],
+    deps = [":magic_enum"],
+)
+
+cc_library(
+    name = "catch",
+    srcs = [],
+    hdrs = ["test/3rdparty/Catch2/catch.hpp"],
+    strip_include_prefix = "test/3rdparty/Catch2",
+)
+
+cc_test(
+    name = "test",
+    srcs = [
+        "test/test.cpp",
+    ],
+    copts = ["-std=c++17"],
+    deps = [
+        ":catch",
+        ":magic_enum",
+    ],
+)
+
+cc_test(
+    name = "test_flags",
+    srcs = [
+        "test/test_flags.cpp",
+    ],
+    copts = ["-std=c++17"],
+    deps = [
+        ":catch",
+        ":magic_enum",
+    ],
+)