Skip scan if file does not exist

- Skip caching timestamps for symbolic links by checking if
`File.exists?(file)`.  This resolves exception:
`Unable to get stat for './src/my_file.cr': No such file or directory (Errno)`

- Update API documentation with `ignore` reference

Author: apmiller108

CRYSTAL_API.md

@@ -60,6 +60,7 @@ process_runner = Sentry::ProcessRunner.new(
   build_args: [], # Array of String
   run_args: [], # Array of String
   should_build: true, # Bool
-  files: [] # Array of String
+  files: [], # Array of String
+  ignore: [] # Array of String
 )
-```
+```

src/sentry.cr

@@ -200,6 +200,12 @@ module Sentry
       File.stat(file).mtime.to_s("%Y%m%d%H%M%S")
     end
 
+    private def ignored?(file : String)
+      @ignore_regexes.any? do |regex|
+        regex === file || regex === file.split("/").last
+      end
+    end
+
     # Compiles and starts the application
     #
     def start_app
@@ -221,7 +227,7 @@ module Sentry
       app_process = @app_process
       files = @files
       Dir.glob(files) do |file|
-        next if @ignore_regexes.any? { |r| r === file || r === file.split("/").last }
+        next if !File.exists?(file) || ignored?(file)
         timestamp = get_timestamp(file)
         if FILE_TIMESTAMPS[file]? && FILE_TIMESTAMPS[file] != timestamp
           FILE_TIMESTAMPS[file] = timestamp