Add option to anonymise output

There has recently been [a discussion](https://wiki.archlinux.org/index.php/Talk:ArchMap#Project_deletion) on the Arch Wiki about whether this project should be abandoned due to the privacy implications.

I'm not sold on the idea just yet and have proposed a middle ground where the username (or possibly everything apart from the location) is anonymised.

This issue intends to work out:
* What data is anonymised
* How that data is then anonymised
* What flag that option would live behind (`--hide`?)

My current thinking is to replace the username with a random hash to ensure backwards compatibility:
```
51.5073219,-0.1276474 "User 0" # London, UK
-33.9289049,18.4172485 "User 1" # Cape Town, South Africa
55.7516335,37.6187042 "User 2" # Moscow, Russia
35.6823815,139.7530053 "User 3" # Tokyo, Japan
45.4210328,-75.6900219 "User 4" # Ottawa, Canada
-15.7934003,-47.8822933 "User 5" # Brasília, Brazil
```
would become
```
51.5073219,-0.1276474 "X4zjsc # London, UK
-33.9289049,18.4172485 "EFmDXb" # Cape Town, South Africa
55.7516335,37.6187042 "MLfFK6" # Moscow, Russia
35.6823815,139.7530053 "b7uGnR" # Tokyo, Japan
45.4210328,-75.6900219 "8huq10" # Ottawa, Canada
-15.7934003,-47.8822933 "hOeVP8" # Brasília, Brazil
```