rewrite readme to showcase new features

[yjunechoe]

Currently just has inspect, capture, modify with ggproto methods. Some things to consider adding/replacing:

Use dplyr to emphasize how ggtrace exposes ggplot2 internals in functional programming terms

• ggplot_build.ggproto selective plotting

  ggtrace(
      method = ggplot2:::ggplot_build.ggplot,
      trace_steps = -1,
      trace_exprs = expr(data[[1]] <- data[[1]] %>% filter(PANEL == 2)),
      verbose = FALSE
  )
  barplot_plot
  

• persistent tracing / global_ggtrace_state()

[yjunechoe]

superseded by #74