add 0 key for beginning-of-line

[nathantypanski]

This adds a shortcut for the 0 key to take the user to the beginning
of a given line, like Vim. It simply calls line_home like ^ does,
but this one is in my muscle memory from Vim usage.

By the way, thanks for developing this. I was looking for a dead-simple
command-line CSV viewer, and this fits the bill.

[nathantypanski]

Ugh. This breaks the modifier key handling for 0. I'll update this PR if I fix it.

[nathantypanski]

Well, I've fixed that issue. Behavior for 0 is now the same as Vim:

• Pressing 0 without any modifier present goes to the beginning of the current line.
• If a modifier is already present, 0 is considered a part of that modifier sequence.

There's some jumping around required because of the fact that .isdigit() fails if c > 256, but it's otherwise straightforward.

Note: It occurs to me that because of this behavior, even in your current upstream pressing any key c > 256 that is not assigned in the Viewer.keys attribute will crash the program. For example, pressing F2 will trigger a crash.

[firecat53]

Thanks for doing that!! I've been hitting 0 too and been meaning to fix that. Haven't touched this for awhile though. Gimme a day or two to look at this (in between other stuff :) )

Scott

[nathantypanski]

Of course!

One other thing I noticed when working on this/reviewing it:

Line 121 is probably unnecessary. But I have something of a habit of changing this kind of "boolean context" usage of regular objects into boolan expressions, since Python's behavior isn't always what you mean to say. For example, even though it works fine for strings:

>>> not ''
True
>>> not '0'
False
>>> not 'False'
False

it gets stranger when you start mixing numerics:

>>> not 0
True
>>> not 99
False
>>> modifier = '{}{}'.format(modifier, '0')
>>> not int(modifier)
True

So I just try to avoid these weird bugs altogether by specifying booleans explicitly when it doesn't hurt the duck typing.

[firecat53]

Thanks! I added one more commit that looks it fixes the crash on unhandled characters.

[nathantypanski]

Good job fixing the crash. Thanks for merging. 💃