fix: remove dashes and underscores that seems to generate errors

[hasecilu]

Fix #440

[gegoune]

I don't have any problem with neither - nor _. I am on 0.10, so perhaps it's different on 0.9? Could someone on stable confirm if those work:

:hi foo-bar gui=bold
:hi foo_bar gui=bold

(Both work on 0.10.)

While / errors.

[hasecilu]

I don't have any problem with neither - nor _. I am on 0.10, so perhaps it's different on 0.9? Could someone on stable confirm if those work:

:hi foo-bar gui=bold
:hi foo_bar gui=bold

(Both work on 0.10.)

While / errors.

Same here but with NVIM v0.9.5, LuaJIT 2.1.1702233742.
For me dash and underscore work and slash fails.

[haakonfp]

on v0.9.4. I have the issues mentioned in #440.

:hi foo-bar gui=bold errors.
:hi foo_bar gui=bold does not error.
:hi foo/bar gui=bold also errors.

[meatballs]

Turns out I was still on 0.9.1 Updating to 0.9.5 solved the problems in #440

[gegoune]

Thanks for the fix @hasecilu.
Let's merge it in, group names look better that way anyway and it fixes backwards incompatibility.