Allow type_alias argument to prefer them over interfaces

[hgiesel]

(see #80)

Adds the type_alias argument to #[tsify] to conditionally generate type aliases instead of interfaces.

I had to reformat a lot of tests to use indented type aliases, because inline types do not support doc comments at all.
So what was previously:

export type NonIdentifierRenameEnum = { "hello-world": boolean; } | { "hel#&*world": number; } | { "hello world": string; } | { "": number; } | { should_not_quote: string; };

Will now become this:

export type NonIdentifierRenameEnum = {
    "hello-world": boolean;
} | {
    "hel#&*world": number;
} | {
    "hello world": string;
} | {
    "": number;
} | {
    should_not_quote: string;
};

Closes #61

[siefkenj]

Sorry, I merged some PRs and now there are some conflicts to clean up...

This idea looks nice, but I find the name type_alias confusing. (To me, I would expect type_alias = "bla bla...")

Maybe convert_as_type or convert_as_interface = false. @madonoharu Did you have any opinion here?

[hgiesel]

Maybe convert_as_type or convert_as_interface = false

How about as_type_alias, or prefer_type_alias. I'd refrain from just calling it type, because an interface is a type too.

[siefkenj]

Let's go with as_type_alias.

This all looks good. Would you be able to add an example to the readme?