Add support for chained epoxy_style_* transformers

[gadenbuie]

Adds an epoxy_style() function that allows you to chain epoxy's style transformers together. In an epoxy chunk (or in glue::glue()) you can pass epoxy_style() to the .transformer argument to apply a series of transformers during the string replacement.

# bold each item and then collapse
glue::glue("{letters[1:3]&}", .transformer = epoxy_style("bold", "collapse"))
#> **a**, **b** and **c**

# Note that order matters, left -> right is inner -> outer
glue::glue("{letters[1:3]&}", .transformer = epoxy_style("collapse", "bold"))
#> **a, b and c**

# epoxy_style() can be string, function symbol, or function call
glue(
  "{letters[1:3]&}",
  .transformer = epoxy_style(
    "code",
    epoxy_style_bold,
    epoxy_style_collapse(last_and = ", ... and ... ")
  )
)
#> **`a`**, **`b`**, ... and ... **`c`**

Fixes #13

[gadenbuie]

Reference docs preview for epoxy_style(): http://pkg.garrickadenbuie.com/epoxy/preview/pr26/reference/epoxy_style.html