title: Merging two dataset - include unique rows
body: <p>I have a dataset as this:</p>
  
  <pre><code>Island,Individual,all1,all2
  Santiago,CVW3,01,01
  Santiago,CVW8,01,02
  Santiago,CVW9,03,03
  Santiago,CVW10,01,01
  Santiago,CVW12,03,03
  Santiago,CVW19,01,01
  Santiago,CVW25,01,04
  Santiago,CVW39,04,04
  Santiago,CVW40,01,01
  Santiago,CVW41,01,01
  Fogo,CVW64,04,04
  Fogo,CVW67,01,01
  Fogo,CVW70,02,04
  Fogo,CVW74,03,05
  Fogo,CVW80,01,02
  Fogo,CVW99,01,04
  Fogo,CVW101,02,02
  Fogo,CVW103,01,02
  Fogo,CVW104,01,04
  Fogo,CVW111,02,04
  SaoNicolau,CVW81,01,01
  SaoNicolau,CVW87,01,01
  SaoNicolau,CVW92,01,01
  SaoNicolau,CVW124,01,01
  SaoNicolau,CVW125,01,01
  SaoNicolau,CVW133,01,01
  SaoNicolau,CVW136,01,01
  </code></pre>
  
  <p>The data in all1 and all2 are factors.
  I need to calculate the combined frequency of each of the factors in all1 and all2, both within each subset of Island and for the overall dataset.
  So for Santiago and 01, the frequency would be: 12/20.</p>
  
  <p>I have worked on splitting the dataset into island as such:</p>
  
  <pre><code>#Get data subset for Santiago:
  stsplit = subset(locus, locus$Island == "Santiago")
  
  allfr1st = count(stsplit, "all1")
  allfr2st = count(stsplit, "all2")
  
  #Rename headers:
  names(allfr1st) = c("Allele", "Freq")
  names(allfr2st) = c("Allele", "Freq")
  </code></pre>
  
  <p>This gives me:
      allfr1st:</p>
  
  <pre><code>Allele,Freq
  01,7
  03,2
  04,1
  
  
  allfr2st:
  
  Allele,Freq
  01,5
  02,1
  03,2
  04,2
  </code></pre>
  
  <p>Now I want to sum the frequencies of each allele for the two data frames, so I would get the below</p>
  
  <pre><code>Allele,Freq
  01,12
  02,1
  03,4
  04,3
  </code></pre>
  
  <p><code>merge()</code> excludes allele 02, as it is not in both datasets.
  How do I combine the two in the right way?</p>
  
  <p>If there is (and there surely is) a more obvious way of getting what I want (the frequencies of 01, 02, 03 etc both overall and within islands), please feel free to give me advice on that as well.</p>