R笔记本编织到pdf超链接颜色不起作用

Question

这是问题的MWE。我包括了urlcolor和linkcolor的选项，但是我的pdf输出没有蓝色链接。当我在keep_tex中将true设置为hyperset时，链接颜色未设置为蓝色。

---
title: "Color links not working"
output:
  pdf_document:
    number_sections: false
    keep_tex: false
  urlcolor: blue
  linkcolor: blue
---

Here is a link to [google](www.google.com).

这是pdf输出。

还有sessionInfo()的输出：

> sessionInfo()
R version 3.4.3 (2017-11-30)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.4 LTS

Matrix products: default
BLAS: /usr/local/lib/R/lib/libRblas.so
LAPACK: /usr/local/lib/R/lib/libRlapack.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8       
 [4] LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C              
[10] LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] dplyr_0.7.6     lubridate_1.7.4 gtrendsR_1.4.2 

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.18       pillar_1.3.0       compiler_3.4.3     plyr_1.8.4        
 [5] bindr_0.1.1        base64enc_0.1-3    tools_3.4.3        digest_0.6.16     
 [9] jsonlite_1.5       evaluate_0.11      tibble_1.4.2       gtable_0.2.0      
[13] anytime_0.3.1      pkgconfig_2.0.2    rlang_0.2.2        rstudioapi_0.7    
[17] curl_3.2           yaml_2.2.0         bindrcpp_0.2.2     stringr_1.3.1     
[21] knitr_1.20         rprojroot_1.3-2    grid_3.4.3         tidyselect_0.2.4  
[25] glue_1.3.0         R6_2.2.2           rmarkdown_1.10     ggplot2_3.0.0     
[29] purrr_0.2.5        magrittr_1.5       backports_1.1.2    scales_1.0.0      
[33] htmltools_0.3.6    assertthat_0.2.0   RApiDatetime_0.0.3 colorspace_1.3-2  
[37] stringi_1.2.4      lazyeval_0.2.1     munsell_0.5.0      crayon_1.3.4