File manager

[godrei]

This PR adds OpenReaderIfExists and ReadDirEntryNames to fileutil.FileManager.

OpenReaderIfExists

Can be used to obtain an io.Reader for a file. There were two motivations behind this function:
1, make file existence test easier:

with the stdlib:

f, err := os.Open(pth)
if err != nil {
  if os.IsNotExist(err) {
    // file doesn't exist
  } else {
    // other error
  }
}

with OpenReaderIfExists:

r, err := filemanager.OpenReaderIfExists(pth)
if err != nil {
   // other error
}
if r == nil {
   // file doesn't exist
}

example: https://github.com/bitrise-io/go-flutter/pull/2/files#diff-aa912ba3e8a7389b3812ef842a8be72c6d7dcda3e0ae4e884a75072e38e72753R13

2, Make testing easier:

filemanager := new(mocks.FileManager)
filemanager.On("OpenFile", "<SOME_PATH>").Return(strings.NewReader(<MOCK_CONTENT>), nil)

ReadDirEntryNames

Supports mocking os.ReadDir.

example usage: https://github.com/bitrise-io/go-flutter/pull/2/files#diff-1878dc8ea63a80f862179a2c298fce835f3ed2952f88b608a5b80f0fe8a30600R74