Run only blocks with specific tags

[rikhuijzer]

Allow the user set a tag to one or more code blocks. For example,

```jl foo
1 + 1
```

```jl foo
2 + 2
```

And only run this code when doing gentag("foo") or gentag(["foo", ...]).

[rikhuijzer]

Or use line numbers/ranges to avoid having to set tags

[rikhuijzer]

Even better: use start and end strings like how it’s easy to find the line which throws an error.

benefit is that strings are more flexible starting points than line numbers

[rikhuijzer]

A Pluto integration would probably be better via PlutoStaticHTML.

[rikhuijzer]

Fixed in #289. It's now possible to run gen(file, block_number). The block number can be found by letting the block throw an error (or manually counting, of course).