flextable example
In flextable: Functions for Tabular Reporting

library(knitr)
library(flextable)
library(officer)
library(magrittr)
library(data.table)
opts_chunk$set(echo = FALSE, message = FALSE)

col_palette <- c("#D73027", "#F46D43", "#FDAE61", "#FEE08B", 
  "#D9EF8B", "#A6D96A", "#66BD63", "#1A9850")
cor_matrix <- cor(mtcars)
mycut <- cut( cor_matrix, 
  breaks = c(-1, -0.75, -0.5, -0.25, 0, 0.25, 0.5, 0.75, 1), 
  include.lowest = TRUE, label = FALSE)
mycolors <- col_palette[mycut]
data <- data.frame(rowname = row.names(cor_matrix), stringsAsFactors = FALSE) %>%
  cbind(cor_matrix)

An example

flextable(data) %>%
  bg(j = colnames(cor_matrix), bg = mycolors) %>%
  align(align = "center", part = "all") %>%
  compose(i = 1, j = 1, value = as_paragraph(""), part = "header") %>% 
  colformat_num(j = colnames(cor_matrix), digits = 2) %>% 
  autofit()

Another example

ft <- flextable(head(iris))

# measure column widths but only for the body part
w_body <- dim_pretty(ft, part = "body")$widths
# measure column widths only for the header part and get the max
# as height value for rotated text
h_header <- max( dim_pretty(ft, part = "header")$widths )

ft <- rotate(ft, j = 1:4, rotation="btlr",part="header")
ft <- rotate(ft, j = 5, rotation="tbrl",part="header")

ft <- valign(ft, valign = "center", part = "header")
ft <- align(ft, align = "center", part = "all")

# Manage header height
ft <- height(ft, height = h_header * 1.1, part = "header")
# ... mainly because Word don't handle auto height with rotated headers
ft <- hrule(ft, i = 1, rule = "exact", part = "header")

ft <- set_caption(ft, caption = "iris dataset", autonum = run_autonum(bkm = "iris"))
ft

flextable - demo as_grouped_data

data_CO2 <- dcast(as.data.table(CO2), 
  Treatment + conc ~ Type, value.var = "uptake", fun.aggregate = mean)
data_CO2 <- as_grouped_data(x = data_CO2, groups = c("Treatment"))

as_flextable( data_CO2 ) %>% 
  bold(j = 1, i = ~ !is.na(Treatment), bold = TRUE, part = "body" ) %>% 
  bold(part = "header", bold = TRUE ) %>% 
  width(width = 1.5) %>% 
  compose(i = ~ !is.na(conc), j = "conc", 
          value = as_paragraph(
            as_chunk(conc, formatter = function(x) sprintf("%.0f", x))
          )
  ) %>% add_footer_lines("dataset CO2 has been used for this flextable") %>% 
  bg(bg = "#FFFFFF", part = "footer") %>% 
  set_header_labels(conc = "Concentration") %>% 
  width(width = c(1.5, 1, 1))