Add user-configurable keybindings

[masmu]

Users can now customize keyboard shortcuts by placing a keybindings file at
~/.config/moor/keybindings (or $XDG_CONFIG_HOME/moor/keybindings).

Usage

Print the built-in defaults as a ready-to-edit starting point:

moor --print-default-keybindings > ~/.config/moor/keybindings

Point moor at a specific file (errors are fatal):

moor --keybindings ~/my-keybindings somefile.txt

File format

# Lines starting with # are comments; blank lines are ignored.
# Format: <key>  <action>
# Keys: a-z, A-Z, 0-9, special chars, ctrl-a…ctrl-z, escape, enter,
#       backspace, delete, up/down/left/right, home, end, pageup/pagedown,
#       alt-up/down/left/right, space
# Use 'noaction' to disable a key.

[viewing]
escape  quit
q       quit
j       scroll-down
k       scroll-up
/       search-forward
# ... one section per mode: viewing, search, filter, goto-line,
#     colon-command, mark, jump-to-mark, input

[input]
ctrl-a  cursor-home
ctrl-e  cursor-end
ctrl-k  delete-to-end

By default the file is overlaid on top of the defaults: only the keys you
list change, everything else stays as-is. Add !clear at the top to start
from a blank slate instead.

Error handling

How the file was found	Parse error	Unknown key/action
--keybindings (explicit)	Fatal error before startup	Fatal error
Default path	Warning to stderr, continue with partial bindings	Warning to stderr

Implementation notes

• All per-mode switch key / switch char blocks are replaced by a
  table-driven dispatch through ModeBindings, with common scroll/quit/edit
  actions factored into a single executeCommonAction method on *Pager.

What doesn't change

Default key bindings are identical to before. Users who don't create a config
file see no difference in behavior.

Related issues

• Fixes Keybindings to less / vim #107
• Fixes search: exit using ^C #419
• Addresses search: readline keybindings support #334

[walles]

I honestly don't know what to do with this.

Reviewing 2810 lines of code is a lot of work, and I'd need to do that since I'm the one who'd have to maintain this if merged.

Any suggestions?

[masmu]

I think it's not too bad. A lot of the added lines are tests (+1311).
In keybindings.go (+663) half of the lines are constants, definitions and keybindings_parser.go (+141) is just one straight forward function.
The rest of the diff is evenly distributed and the numbers for additions and removals almost cancel each other out. So the bare numbers look more terrifying than it actually is.
And all the pagermode-*.go do not look much different from what they were before.

My motivation comes from still looking for a not ancient pager with key binding support. That is a feature with a bigger audience then people might think, probably people like me who never got over / for starting a search. It feels like you are screwed just by not using an English keyboard. And this affects potentially a lot of people, so this is also a chance to grow the project.

You might want to start by looking at keybindings.go, then pager.go and finally just all the pagermode-*.go files to see if you like the approach and the code.