[Feature Request] Generalize text highlight to text styles

[byxrolland]

The latest version only allows user to setup background colors for text highlight using "Alternative Highlight Colors". However, I find such style of highlighting is not very needed for matching texts in my daily work. Most of the time, background coloring is too strong for emphasizing texts and can collide with line highlighting, making the scene chaotic. Usually, if I want to mention a word, I will just highlight the lines and use comments to explain the details.

Thus, I suggest letting user create styles to customize the style entirely for matching texts (and lines), including font weight, font shape, font family, font color, underline, and strike-through line.

For example, I can set up a "placeholder" style ph to override syntax highlighting, making the texts grey and italic but doesn't affect anything else.

Take a look at the code

```latex pht:<:>
\DTLsort[<replacement keys>]{<sort criteria>}{<database name>}
```

It is very self-explanatory that texts like <replacement keys> are placeholders. It completes the syntax highlighting since placeholders are not of the grammar. Background highlighting doesn't work this way.

Also, strike-through line is also self-explanatory. It can tell readers that certain keywords should be deleted, like const.

This feature can also work for inline codes.

[mugiwara85]

@byxrolland That is actualy a pretty awesome idea! I'm already working on it. I get back to you later, because I would really appreciate if you could test it thourughly.

[byxrolland]

I'm look forward to it.

Also, please allow double quotes to be used as hint for highlighting. Otherwise, multiple-word hints can't be used.

For example, if I want to highlight all the variables that of the type const & and all return statements that return nothing, I can write like this:

```cpp hl:"const &","return;"
int const a;
int const & b = a;   // highlighted
return a;
return;         // highlighted
```

However, with the current mechanism:

```cpp hl:const &,return;
int const a;        // highlighted
int const & b = a;   // highlighted
return a;
return;
```

It gives me a wrong answer.

[mugiwara85]

You added too many "-s.

Isn't this what you want: ```cpp hl:"const &,return;" ? This highlights const & and return.
after any parameter if you want to specify a string that contains a space you have to put the full parameter value int "-s.

[byxrolland]

I'm not sure lumping all keywords into a single pair of quotes, separated by ,, is a good idea. It can create unexpected errors.

For example, if I only want to highlight functions that take both a and b, the result is incorrect:

```cpp hl:"a, b"
f(a);    // false positive
g(b);    // false positive
h(a, b);
a = j(b);  // false positive
```