title: Write xts/zoo object to csv with index
body: <pre><code>&gt; library(PerformanceAnalytics)
  &gt; data(managers)
  &gt; class(managers)
  [1] "xts" "zoo"
  &gt; head(managers)
                HAM1 HAM2    HAM3    HAM4 HAM5 HAM6 EDHEC LS EQ SP500 TR US 10Y TR US 3m TR
  1996-01-31  0.0074   NA  0.0349  0.0222   NA   NA          NA   0.0340   0.00380  0.00456
  1996-02-29  0.0193   NA  0.0351  0.0195   NA   NA          NA   0.0093  -0.03532  0.00398
  1996-03-31  0.0155   NA  0.0258 -0.0098   NA   NA          NA   0.0096  -0.01057  0.00371
  1996-04-30 -0.0091   NA  0.0449  0.0236   NA   NA          NA   0.0147  -0.01739  0.00428
  1996-05-31  0.0076   NA  0.0353  0.0028   NA   NA          NA   0.0258  -0.00543  0.00443
  1996-06-30 -0.0039   NA -0.0303 -0.0019   NA   NA          NA   0.0038   0.01507  0.00412
  </code></pre>
  
  <p>When I try to export this object to csv using <code>write.csv</code>, the date indices are not exported. How do I enforce the first column of the csv output to be the date indices?</p>
  
  <pre><code>"","HAM1","HAM2","HAM3","HAM4","HAM5","HAM6","EDHEC LS EQ","SP500 TR","US 10Y TR","US 3m TR"
  "1",0.0074,NA,0.0349,0.0222,NA,NA,NA,0.034,0.0038,0.00456
  "2",0.0193,NA,0.0351,0.0195,NA,NA,NA,0.0093,-0.03532,0.00398
  "3",0.0155,NA,0.0258,-0.0098,NA,NA,NA,0.0096,-0.01057,0.00371
  "4",-0.0091,NA,0.0449,0.0236,NA,NA,NA,0.0147,-0.01739,0.00428
  "5",0.0076,NA,0.0353,0.0028,NA,NA,NA,0.0258,-0.00543,0.00443
  "6",-0.0039,NA,-0.0303,-0.0019,NA,NA,NA,0.0038,0.01507,0.00412
  "7",-0.0231,NA,-0.0337,-0.0446,NA,NA,NA,-0.0442,-0.001,0.00454
  "8",0.0395,-1e-04,0.0461,0.0351,NA,NA,NA,0.0211,-0.00448,0.00451
  "9",0.0147,0.1002,0.0653,0.0757,NA,NA,NA,0.0563,0.02229,0.0047
  "10",0.0288,0.0338,0.0395,-0.018,NA,NA,NA,0.0276,0.02869,0.00428
  "11",0.0156,0.0737,0.0666,0.0458,NA,NA,NA,0.0756,0.02797,0.00427
  "12",0.0176,0.0298,0.0214,0.0439,NA,NA,NA,-0.0198,-0.02094,0.00442
  </code></pre>