Correct ligature issues when styling is applied to syllable

Corrected interaction issues between text styles and ligatures.  The LaTeX commands like `\textit` insert italics correction, preventing ligatures being formed between their arguments if two occur sequentially.  On the other hand the switches like `\itshape` do not.  Since gregorio breaks up syllables around the vowel and then applies the formating commands to each part, this behavior showed up.  We switch to using the switches to avoid this (except for underlining, for which a switch does not exist).  See [gregorio-project#1444](gregorio-project#1444).
A fix for color is still in the works.

CHANGELOG.md

@@ -3,6 +3,8 @@ All notable changes to this project will be documented in this file.
 As of v3.0.0 this project adheres to [Semantic Versioning](http://semver.org/). It follows [some conventions](http://keepachangelog.com/).
 
 ## [Unreleased][CTAN]
+### Fixed
+- Corrected interaction issues between text styles and ligatures.  The LaTeX commands like `\textit` insert italics correction, preventing ligatures being formed between their arguments if two occur sequentially.  On the other hand the switches like `\itshape` do not.  Since gregorio breaks up syllables around the vowel and then applies the formating commands to each part, this behavior showed up.  We switch to using the switches to avoid this (except for underlining, for which a switch does not exist).  We also fix this for color tags by loading `luacolor` which changes how the `\color` tag is implemented to allow ligatures to span groups.  See [#1444](https://github.com/gregorio-project/gregorio/issues/1444).
 
 
 ## [5.2.1] - 2019-04-06

tex/gregoriotex.sty

@@ -102,22 +102,22 @@
 %%%%%%%%%
 
 \def\GreItalic#1{%
-  \textit{#1}%
+  {\itshape #1}%
   \relax %
 }
 
 \def\GreSmallCaps#1{%
-  \textsc{#1}%
+  {\scshape #1}%
   \relax %
 }
 
 \def\GreBold#1{%
-  \textbf{#1}%
+  {\bfseries #1}%
   \relax %
 }
 
 \def\GreTypewriter#1{%
-  \texttt{#1}%
+  {\ttfamily #1}%
   \relax %
 }