Add placeholder to numeric_input

[osenan]

Short description (with a reference to an issue).

This PR closes the following issue: #428

It includes a change in the tag creation of the numeric_input Additionally, the checks that the placeholder type is character. It also has the updated unit tests to check as well the placeholder argument.

This app creates a placeholder as value in the numeric_input is NULL

library(shiny)
library(shiny.semantic)
ui <- semanticPage(
  numeric_input("ex", "Select number", value = NULL, placeholder = "type value")
)
server <- function(input, output, session) {}
shinyApp(ui, server)

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