Support using conventional commits or alternative messages for the Git publish commit

Prerequisites

[x] Put an X between the brackets on this line if you have done all of the following:
- Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
- Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
- Checked the FAQs on the message board for common solutions: https://discuss.atom.io/c/faq
- Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=is%3Aissue+user%3Aatom
- Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages

Description

In publish.coffee, the message is hard-coded to use Preparing %s release. However, if I'm using conventional commits and commitlint (and associated tooling), the resulting message should be closer to chore(release): preparing v%s release.

Since the publish process is (relatively) atomic, there is no way of interrupting it to have the proper message before it is pushed up and the rest of the logic goes through.

Steps to Reproduce

Use conventional commits.
apm publish minor

Expected behavior:

I expect to have the ability to override or reformat the message.

Actual behavior:

The message is hard-coded.

Reproduces how often: 100%

Versions

Linux, but this is related to how the code works.

Atom    : 1.35.0-beta0
Electron: 2.0.16
Chrome  : 61.0.3163.100
Node    : 8.9.3

apm  2.1.3
npm  6.2.0
node 8.9.3 x64
atom 1.35.0-beta0
python 2.7.15+
git 2.19.1

Additional Information

I like having the ability to track changes, generating change logs, and moving toward the semantic releases. Conventional commits help with that, in my opinion, which is why I've been migrating various libraries over.

atom / apm