* Add option to install OMZ without replacing .zshrc

tools/install.sh respects REPLACE_RC environment variable
--noreplace-rc flag sets REPLACE_RC='no'

* Change REPLACE_RC=no to KEEP_ZSHRC=yes

Change --noreplace-rc to --keep-zshrc

Fixes #8665

Fixes #8676

Provides a different solution to #8332 and #8333

Add random theme and consolidate logic from init and themes plugin

Co-authored-by: Fran Garcia <fran.miranda@gmail.com>

Also add comments and unset leftover variables, and print only the
name of the theme loaded.

When looking for $ZSH_CUSTOM themes, the chosen algorithm is to add
the theme names to the pool disregarding the path, and then source
whatever theme is selected with the same logic as the init script,
which is to source first custom themes even if there is another
default theme of the same name.
Co-authored-by: Mihai Serban <mihai.serban@gmail.com>

The statements for selecting a random theme in oh-my-zsh.sh and the themes
plugin are duplicate. Most people eventually settle on a theme, making those
lines in oh-my-zsh.sh superfluous. To address those, it may makes sense to put
the random theme functionality into a theme of its own (since themes are just
zsh scripts.

Fixes #8637

* Change indentation to 2 spaces in prompt_bzr function
* Check if in a bzr repository and optimize bzr calls in prompt_bzr

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

Fixes #8081

- Similarly to the mvn plugin
- Without this fix, the shell crashes in some cases

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

Closes #4726
Closes #4774
Closes #8275