Provide an option to generate BUILD.bazel

[bshashank]

In my project, I already have files name build, so I'd like to generate BUILD.bazel files via pazel. This PR adds a new option:
BUILD_FILE_NAME = 'BUILD.bazel'
and setting that allows all the generated files to be named as BUILD.bazel instead of the default BUILD.

Bonus: update gitignore to ignore all bazel-* symlinks

This PR fixes #21

Testing Done:

❯ bazel test //pazel/...
INFO: Analyzed 15 targets (0 packages loaded, 0 targets configured).
INFO: Found 10 targets and 5 test targets...
INFO: Elapsed time: 0.210s, Critical Path: 0.01s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
//pazel/tests:test_bazel_rules                                  (cached) PASSED in 0.4s
//pazel/tests:test_generate_rule                                (cached) PASSED in 0.1s
//pazel/tests:test_helpers                                      (cached) PASSED in 0.1s
//pazel/tests:test_parse_imports                                (cached) PASSED in 0.1s
//pazel/tests:test_pazel_extensions                             (cached) PASSED in 0.1s

INFO: Build completed successfully, 1 total action