refactor: use conf.d for handling the fish_prompt event

[jorgebucaran]

This PR fixes a problem when uninstalling pure where the function pre_prompt is still defined (can't be erased because it's not in its own fish file—event handlers should go in conf.d), which continues to handle fish_prompt events. At this point in time, pure's helper functions have been erased.

fish: Unknown command '__parse_current_folder'
~/.config/fish/functions/fish_prompt.fish (line 1): 
__parse_current_folder
^
in command substitution
	called on line 101 of file ~/.config/fish/functions/fish_prompt.fish

in function 'pre_prompt'
	called on standard input

in event handler: handler for generic event 'fish_prompt'

fish: Unknown command '__parse_git_branch'
~/.config/fish/functions/fish_prompt.fish (line 1): 
__parse_git_branch
^
in command substitution
	called on line 144 of file ~/.config/fish/functions/fish_prompt.fish

in function 'pre_prompt'
	called on standard input

in event handler: handler for generic event 'fish_prompt'

Additionally, it moves the installer.md and installer.fish files to their own tools/ directory closing #83.

cc @rafaelrinaldi @edouard-lopez

[jorgebucaran]

@rafaelrinaldi @edouard-lopez Is there anything still to do to get this merged? Can I help in some way?

[edouard-lopez]

@jorgebucaran I'm ok with the change however the tests CI is broken in relation to fisher v3 I reckon.

[jorgebucaran]

@edouard-lopez Yes, indeed. I’ve just finished rewriting Fisher and Fishtape is coming up next.

[edouard-lopez]

I fixed the CI on master with 84999c9

[edouard-lopez]

@jorgebucaran the CI still fails, it doesn't run on the latest version of master, could you rebase your PR?

[jorgebucaran]

@edouard-lopez Rebased.

[edouard-lopez]

@jorgebucaran Cool, now tests fail in a meaningful way:

source: Error encountered while sourcing file '/home/travis/build/rafaelrinaldi/pure/tests/../installer.fish':
source: No such file or directory

First line of tests/installer.test.fish need to be updated to:

source $DIRNAME/../tools/installer.fish

I submitted a commit on your PR directly 👍

[jorgebucaran]

@edouard-lopez Ah, my bad. 🙇

Thanks!