Don't override the users indentation and textwidth

[ghost]

Vim plugins shouldn't override user settings unless they ask!

Stops the plugin from modifying the users settings by default
instead makes them opt-in with g:rust_recommended_style

[reem]

I actually expect a language file to override the indentation etc. of files in that language. I'm in favor of adding a switch, but I think the default should be flipped in this PR.

[ghost]

Personally, I find it confusing when a plugin overrides my settings. If I set textwidth=80 in my .vimrc, I don't expect that to get changed as soon as I open a rust file. I only expect the plugin to overide my settings if it's required by the file's syntax, such as using tabs in makefiles.

There doesn't seem to be a convention that vim syntax files follow here, but I'd say it makes sense to go by the princple of least suprise. What do other vim users think?

[soltanmm]

I personally like my plug-ins telling me what the 'standard' way of formatting is, and then letting me decide after I've become aware of that information whether I'm okay with it or not. I'd rather the technology I use having opinions (esp. 'standard' opinions) that I can take into account than allowing me to blindly charge in.

[ghost]

Updated setting to be enabled by default.

[bstrie]

Agreed that having a switch here is good.