Add item.id to use as syntax lookup widget slug

[kevanstannard]

Suggestion for #162

The GithubSlugger seems to convert non alphanumeric characters to hyphens.

For example (.) => {} is converted to --.

Which results the URL /syntax-lookup#--

What do you think about introducing an id field into the items? This would need to be unique for each item, but would provide a convenient field for converting to a slug.

Which would then enable URLs such as /syntax-lookup#uncurried-function

[ryyppy]

Yeah that's better.
Note we will most likely extract that meta data into the frontmatter of each doc file instead (just so you know)