Nu Shell Updates - Week #18

1st chunk of `blog/2019-12-27-twin0018.md`

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# This week in Nu #18

Ho ho ho happy holidays!

- andrasio continues work on the subcrate refactor ([#1128](https://github.com/nushell/nushell/pull/1128))
- jonathandturner fixed an issue with utf-8 support ([#1126](https://github.com/nushell/nushell/pull/1126))
- jonathandturner also added a new `$nu` variable to replace `$nu:path` and related. Additionally, `ps` now has a new memory usage column ([#1112](https://github.com/nushell/nushell/pull/1112))
- notryanb helped update our docs with the new `$nu` variable ([#1125](https://github.com/nushell/nushell/pull/1125))
- quebin31 fixed calling `cd` by itself ([#1124](https://github.com/nushell/nushell/pull/1124))
- thegedge fixed a performance bug with pipelines between external commands ([#1108](https://github.com/nushell/nushell/pull/1108))

Title: Nu Shell Updates - Week #18

Summary

This week's updates to the Nu shell include ongoing subcrate refactoring, improvements to UTF-8 support, the addition of a new `$nu` variable, a memory usage column in `ps`, documentation updates, a fix for the `cd` command, and a performance fix for pipelines between external commands.