Add x64 support.

**DO NOT MERGE**


TODO:
- [ ] sort the installer
- [ ] do we need both NHunspellx86.dll and NHunspellx64.dll in the final builds?
- [ ] see if we can find a workaround for `ngen` failing for x64

@niksedk: please check the above question and let me know. **DO NOT MERGE**, the pull request is open for discussion at this point.