Split apply logic by fragment type (#32)

Remove the Applier type and replace it with TextApplier and
BinaryApplier, both of which operate on fragments instead of on full
files. Move the logic that previously existed in Applier.ApplyFile to
the top-level Apply function.

Also restructure arguments and methods to make it clear that appliers
are one-time-use objects. The destination is now set when creating an
applier and the Reset() method was replaced by Close().

.golangci.yml

@@ -11,6 +11,7 @@ linters:
     - golint
     - govet
     - ineffassign
+    - misspell
     - typecheck
     - unconvert
     - varcheck

README.md

@@ -29,7 +29,7 @@ if err != nil {
 
 // apply the changes in the patch to a source file
 var output bytes.Buffer
-if err := gitdiff.NewApplier(code).ApplyFile(&output, files[0]); err != nil {
+if err := gitdiff.Apply(&output, code, files[0]); err != nil {
     log.Fatal(err)
 }
 ```