Fix callback outputs with short ID's

[HammadTheOne]

Closes #278

This PR fixes #278 by doing a regex substitution of the callback output ID's instead of a substring splice.
This PR also adds an additional check for duplicate output ID's before adding callbacks to the callback_map.

@daattali

[daattali]

Is this also a fix for #279 ?

[daattali]

May as well include the output_id in the message, no?

[daattali]

I assume you verified this doesn't cause an error when there's one callback with output ID x and another callback with multiple IDs x and y

[HammadTheOne]

Is this also a fix for #279 ?
Yes, this should address that issue as well.

I assume you verified this doesn't cause an error when there's one callback with output ID x and another callback with multiple IDs x and y

Do you mind clarifying this? If the output ID is duplicated (whether in a single or multiple output callback), it would throw an error, as it does with other backends as well.

Eg.

app %>% set_layout(
  dccInput("text", "test"),
  div(id = "out"),
  div(id = "out2")
)

app %>% add_callback(
  outputs = output("out", "children"),
  params = input("text", "value"),
  function(data) data
)

app %>% add_callback(
  outputs = list(output("out", "children"),
                 output("out2", "children")),
  params = input("text", "value"),
  function(data) paste(data, "2")
)

[daattali]

I didn't realize this should throw an error! In that case ignore my comment :) I thought that only the complete set of outputs cannot be duplicated, I didn't realize a single output cannot appear in multiple callbacks

[daattali]

I don't quite understand what's happening here, but we're sure this is safe -- it doesn't cause issues when two output IDs are similar and differ by just one character -- right

[HammadTheOne]

This is a safer than what we had originally, which constrained output_id to be > 3 characters.

Now that we're simply gsub'ing the 2 dots necessary for the multiple output syntax instead of substringing them, it's safer, since the only constraint is not including periods in the ID, which has already been documented.