New option type `form`

Some tasks allow you to pass edn forms for evaluation. On the CLI you can pass those as string and we'll `read-string` them. If you pass a form in a string in Clojure code it will be interpreted as a string essentially resulting in a no-op. See #666 for details.

To prevent users from running into this issue a warning has been added in #666 but this only covers the :eval option of the repl task leaving any other `edn` options that expect forms as-is.

I think ideally there would be a specific type for options that expect forms (~= function invocations) to distinguish the case of passing data vs. passing a Clojure form.

The [Task Options DSL wiki page](https://github.com/boot-clj/boot/wiki/Task-Options-DSL#types) also mentions a `code` type but that implies evaluation of the form. As I understand this is most often used to pass symbols?

#### Preliminary todo list:

- [ ] extend `boot.cli/parse-atom` by a `form` option:

form (fnil read-string "nil")
- [ ] extend `boot.cli/assert-atom` by a `form` option:

form list?
- [ ] update docs on [Task Options DSL wiki page](https://github.com/boot-clj/boot/wiki/Task-Options-DSL#types)

#### Tasks affected

- `repl` `:eval`

Start in boot.cli at parse-atom and assert-atom, using the issue's proposed form behavior as the starting point. Then review the Task Options DSL wiki page's types section and the repl :eval handling. Done means the form option is supported consistently and the documented option type explains its use.

Written by the indexing model from the issue text.