Add Linux ARMv7 support

[malept]

Have you read the section in CONTRIBUTING.md about pull requests?

Yes

Summarize your changes:

Electron has had Linux ARM support since 0.29.0, but I have been hesitant to merge any PRs for it until Electron properly tagged its downloads with the correct version of the ARM processor that it supports¹. In this case, ARMv7. Fortunately, this is now on the horizon.

However, according to the Electron PR, the properly named ARM builds will only be backported to versions >= 1.0.0. We will support Electron versions 0.29.0 and above by modifying the arguments passed to electron-download to use the old name when the Electron version is less than 1.0.0. This means that a new dependency is required: semver, which checks what Electron version is being specified.

_Blocked by the release of electron/electron#6986_ (cc @kevinsawicki 😄).
Fixes #106.
Closes #107, #423.

Are your changes appropriately documented?

Added documentation about the new supported arch to the CLI and API docs, readme, and NEWS.

Do your changes have sufficient test coverage?

Updated the relevant multitarget tests.

Does the testsuite pass successfully on your local machine?

Yes

¹: It didn't help that the PRs in question didn't have corresponding tests/docs.

TODO

• Fix superfluous download option warnings
• Uncomment semver code (and use semver.lt)

[Gitelaus]

Looks like electron just pulled electron/electron#6986 so I assume this is now plausibly unblocked?

[malept]

Nope, I can't continue work until they actually release a new version, including backporting the old 1.0+ versions per the description of the blocking PR. This is why I amended the PR description earlier this morning to add "the release of", which is different from merging.

[Gitelaus]

@malept ah okay, my apologies! I think I just got a little too excited and jumped the gun. I'll take the chance to say thanks for all the work you've put in thus far.