Do not fail on non-"meta" attributes

[CreepySkeleton]

Attributes are no longer ought to be in the "meta" form.

For example, this is absolutely valid code:

use getset::CopyGetters;
use clap::{Clap, crate_version};

#[derive(Clap, CopyGetters)]
#[clap(version(crate_version!()))]
pub struct Config {}

but your crate fails to parse this with

error: proc-macro derive panicked
 --> src\main.rs:6:16
  |
6 | #[derive(Clap, CopyGetters)]
  |                ^^^^^^^^^^^
  |
  = help: message: attribute: Error("expected `,`")

error: aborting due to previous error

This PR resolves the issue.

[saschagrunert]

LGTM

[Hoverbear]

Thanks! Looks great. :)

[Hoverbear]

Hi! I'm reverting this and #48 since they were merged without having run on CI. I still want to merge thisi PR so please just reopen it and once it passes CI I'm happy to merge it!

[CreepySkeleton]

Wouldn't it be easier to just merge #49 ?

[CreepySkeleton]

I contains a fix.

[Hoverbear]

Right, but master is supposed to build green, and I can't merge the CI fix in #52 unless the tests are passing, and #47/#48 broke the tests. So they need to be reverted, the CI fix needs to be merged, then they can be merged when they pass the tests.