Make sure shiny app objects are not cacheable using asis_output(..., cacheable = FALSE)

This commit reverts d08a2507fa4e51860ea136f96074bf87013cdc42
Make meta of length 0 when we are sure there are no dependencies

knitr::asis_output() will decide whether x is cacheable by checking length(meta) == 0. Some shiny tags are cacheable, such as numericInput(), because it is pure HTML without dependencies, whereas selectizeInput() is not cacheable due to the dependency on selectize.js.
Merge pull request #453 from yihui/feature/nocache

Prevent caching conditionally