Add `commonInput` feature

[krystian8207]

The functionality allows to merge multiple input controllers into one input sent to the server.

Sample usage:

1. commonInputs - merge multiple inputs at once

commonInputs(id = "val",
  selectInput("subval_1", ..., choices = letters, selected = "a"),
  sliderInput("subval_2", ..., min = 0, max = 10, value = 2)
)

which send to the server:

input$val == list(subval_1 = "a", subval_2 = "2")

Extra arguments:

• block - if TRUE blocks sending sub-inputs to the server (using input$subval_1 will have no effect on app reactivity).

2. commonInput - merge each input controller separately marked with common identifier

commonInput(id = "val",  selectInput("subval_1", ..., choices = letters, selected = "a")),
hr(),
commonInput(id = "val",   sliderInput("subval_2", ..., min = 0, max = 10, value = 2))

So you may mark inputs in different application places.

Extra arguments:

• block - same as above but applied to each input sepearately.