Combine semi-active forks into one

[vrybas]

Greetings,

It looks like this fork is the most active one.

In this PR I've merged valuable code from onemastartup fork, fixed
few bugs myself, and resolved conflicts with this fork.

Summary of changes

Highlighting

Use Vim standard highlighting groups. No need to use variables and
UpdateHighlighting() calls. Colors can be set like this

".vimrc
  highlight SignLowComplexity     guifg=#009900 guibg=NONE ctermfg=2 ctermbg=NONE
  highlight SignMediumComplexity  guifg=#bbbb00 guibg=NONE ctermfg=3 ctermbg=NONE
  highlight SignHighComplexity    guifg=#ff2222 guibg=NONE ctermfg=1 ctermbg=NONE

Works for me. If there's any issues, I guess they can be addressed in
the future.

Enable/Disable/Toggle functions

You can also turn flog off and on:

* Turn on flog
    `:call FlogEnable()`

* Turn off flog
    `:call FlogDisable()`

* Toggle flog
    `:call FlogToggle()`

I've resolved conflicts in favor of Flog*() function names, because
they show right away from which plugin function comes from and easier
accessible with autocomplete, like :call Fl<Tab>

RubyParser

The whole complex method process_defs(exp) is gone and replaced with
use of RubyParser, which is used by Flog itself. It reduces
parsing blow-ups, but not all of them. There are some rare cases left.

Add ability to hide low and middle complexity levels

@alexbel awesome contribution. I've cherry-picked it and slightly
changed behaviour of how signs get 'placed' and 'unplaced'.

Unplace signs only in current buffer

Commit 784835d introduced a bug when you got multiple buffers opened in
multiple windows. See bed787a

Put dummy FlogDummySign before setting real signs

This is what vim-gitgutter also does. Without this the sign column
jumps back & forth when show_complexity() is executed and there's
no any signs to show.

So the whole plugin need to be refactored and there are still a few
bugs, but this can be a good start of it's new life.

[alexbel]

That's a lot of work! Good job.
However, I'm not sure that removing the original author (@skammer) is a good idea.

Highlighting. From user's point of view it's much easier to set let g:flog_low_color=#color which is much readable than long line.

FlogEnable() calls ShowComplexity which calls show_complexity which would work for any type of file. I didn't check that maybe I'm wrong.

[vrybas]

@alexbel,

I'm not sure that removing the original author (@skammer) is a good
idea.

Yes, absolutely. Reverted this line.

FlogEnable() calls ShowComplexity which calls show_complexity which
would work for any type

There's ignore_files regex in ShowComplexity() function, which
ignores certain types of files. Need to be extracted to an option, and
maybe moved out from ShowComplexity(). But I'd prefer to
discuss it in next PR.

it's much easier to set let g:flog_low_color=#color

Hmm, highlight groups are meant to be used for this kind of tasks.
I guess most of plugins, which play with colors, rely on highlight
groups, which can be reloaded by user in his config. GitGutter,
Fugitive to name a few.

But I'm ok with both. Let's gather some more opinions though.