Support string replace at sync configuration

[zzjin]

If there would have feature like replace content when sync.

For example:
base repo have one readme file:
A-repo/README.md

![](./CONTRIBUTING.md)

and with synced to B repo, but under a sub direct using config yaml like this:

B/repo:
  - source: README.md
    dest: repoA/docs/README.md

We need to change content inside readme.md ./CONTRIBUTING.md => ../../CONTRIBUTING.md (witch refers to B repo's CONTRIBUTING guide.

At this situation, we cannot use templates because A-repo's README file is its repos main file, and github do not support any templates with md file.

So we want some content replacement feature, to support add relative paths and/or change image link from relative to absolute (CDN) url:

B/repo:
  - source: README.md
    dest: repoA/docs/README.md
    replace:
    - src: ./CONTRIBUTING.md
      dest: ../../CONTRIBUTING.md
    - src: ./img/intro.png
      dest: https://cdn.repoa.com/img/intro.png