Add corpusSize function

Resolves #2

This is a bit of a hack, but it does work with old derekovecs servers.

Change-Id: I1c8f81c7341f0df71012f5991bde5edfc83dc2b0
diff --git a/tests/testthat/test-derekovecs.R b/tests/testthat/test-derekovecs.R
index 7fd4980..d007f4e 100644
--- a/tests/testthat/test-derekovecs.R
+++ b/tests/testthat/test-derekovecs.R
@@ -38,3 +38,7 @@
   expect_true(freq %>% is.numeric & freq > 0)
 })
 
+test_that("corpusSize works", {
+  size <- corpusSize()
+  expect_true(size %>% is.numeric & size > 1000)
+})