Make help string of custom options better readable.

Lets say you support a custom option that reads a
std::pair<int, float> with a default value. Then
you use the function set_custom_option("INT FLOAT", 2)
or something like this. This results in a help string
that looks like "-p INT FLOAT=(1, 0.00000) x 2 Some text".
This is highly confusing if you just want to use the
program.

This commit adds a flag to CLI::Option that can be set
via set_custom_option that turn off all additions to the
description. So set_custom_option("INT FLOAT", 2, true)
results in something like "-p INT FLOAT Some text".

include/CLI/Option.hpp

@@ -182,6 +182,9 @@ class Option : public OptionBase<Option> {
     /// The number of expected values, 0 for flag, -1 for unlimited vector
     int expected_{1};
 
+    /// A flag to signal if the help string needs special treatment.
+    bool custom_{false};
+
     /// A private setting to allow args to not be able to accept incorrect expected values
     bool changeable_{false};
 
@@ -371,6 +374,9 @@ class Option : public OptionBase<Option> {
     /// The number of arguments the option expects
     int get_expected() const { return expected_; }
 
+    /// True if the option is a custom option and needs special treatment for the help string
+    int get_custom() const { return custom_; }
+
     /// True if this has a default value
     int get_default() const { return default_; }
 
@@ -447,12 +453,14 @@ class Option : public OptionBase<Option> {
         if(get_expected() != 0) {
             if(!typeval_.empty())
                 out << " " << typeval_;
-            if(!defaultval_.empty())
-                out << "=" << defaultval_;
-            if(get_expected() > 1)
-                out << " x " << get_expected();
-            if(get_expected() == -1)
-                out << " ...";
+            if(!get_custom()) {
+                if(!defaultval_.empty())
+                    out << "=" << defaultval_;
+                if(get_expected() > 1)
+                    out << " x " << get_expected();
+                if(get_expected() == -1)
+                    out << " ...";
+            }
         }
         if(!envname_.empty())
             out << " (env:" << envname_ << ")";
@@ -584,9 +592,10 @@ class Option : public OptionBase<Option> {
     ///@{
 
     /// Set a custom option, typestring, expected; locks changeable unless expected is -1
-    void set_custom_option(std::string typeval, int expected = 1) {
+    void set_custom_option(std::string typeval, int expected = 1, bool custom = false) {
         typeval_ = typeval;
         expected_ = expected;
+        custom_ = custom;
         if(expected == 0)
             required_ = false;
         changeable_ = expected < 0;