add some strategically placed .gitignore files

because of the implementation of the termux-android Makefile installer,
the .config directory contents are symlinked into ~/.config. this
results in programs being able to write into the repo. these .gitignores
combat this.

.config/.gitignore

@@ -0,0 +1,2 @@
+.config
+rbw

.config/bash/.gitignore

@@ -0,0 +1 @@
+hist

.config/elinks/.gitignore

@@ -0,0 +1,3 @@
+bookmarks
+globhist
+gotohist

.config/nvim/.gitignore

@@ -0,0 +1 @@
+plugin