Support flashing to partitions other than factory

[bugadani]

Resolves #111

Hoping this is sufficient and correct. This PR changes valid partition table criteria to accept the partition table if any partition exists with the "app" type. I'm not sure if app with subtype test should be rejected here.

[bugadani]

Opened as draft because I haven't tried the built executable yet. Edit: seems to work. This PR also adds .vscode to the gitignore file for my convenience. Let me know if I should remove that.

[icewind1991]

instead of accepting partition tables without factory it might make more sense to add a "target partition" option to determine which app partition to flash to

[bugadani]

That might be an improvement, but I'd like to keep a "just pick one for me" option. esp-idf seems to do this, although it can lead to some weirdness: the active ota firmware might be on the second partition, and then flashing the first partition would be silently ineffective.

There is certainly room for improvement here, I've only implemented what I needed right now.

Also note that, as stated in the issue, in my use case there is no factory partition, so I need espflash to accept such partition tables.

[jessebraham]

Thanks for this, I think it is sufficient. We can expand on it in the future if necessary. I was mistaken initially in thinking that the factory partition was required, so this is closer in line with the documentation.

If you could please resolve the one conflict then we can get this merged.